Apparatus for drawing plastic film in a tenter frame

ABSTRACT

Apparatus and method for producing a novel drawn film by propelling individual carriages along opposed loops from a first speed abutted in stacks in carriage collection sections to a second speed space apart in a drawing section of a tenter frame and to a third speed in stack forming sections where the carriages return to the first speed in the stacks. First primaries positioned adjacent one part of each loop develop electromagnetic waves for engaging synchronous secondaries attached to active carriages to provide controlled spacing of such carriages, and second primaries adjacent another part of the loop develop other electromagnetic waves for engaging hysteresis secondaries attached to active and to passive carriages to provide controlled abutting of the carriages.

This is a division of application Ser. No. 07/209,910, filed June 22,1988.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The field of art to which this invention pertains is tenter frames fordrawing plastic films and, more particularly, it is directed to methodsand apparatus for drawing such films using linear motors.

Specifically, this invention involves drawing film by propelling activecarriages, with tenter clips attached, with synchronous linear motors.These same motors are also used to propel stacks of these carriages, atcontrolled speeds, in collection sections prior to entering the tenterframe. Hysteresis linear motors are used to propel the carriages intothe stacks, and further provide a means of propelling passive carriageswith idler clips attached along the return sides of the tenter frame,after completion of the drawing operation.

2. Description of the Related Art

As just indicated, the method and apparatus of this invention areprimarily used to stretch or draw a web of sheet material or film in atenter frame. The film is drawn in the machine direction (MD) bypropelling tenter clips, which grip the film, in pairs along opposedtracks at ever increasing velocities to space the pairs of clips fromeach other and thereby draw the film longitudinally. Transversedirection (TD) drawing occurs as the clips follow diverging portions ofthe tracks.

Typical methods for drawing film in this manner are shown in U.S. Pat.No. 3,890,421 to Habozit; in Japanese patent publication 48-38779; and,in the French patent 2,317,076. These patents and publication, however,do not teach the carefully coordinated controls required, in drawingfilm in accordance with the instant invention. In such invention, pairsof clips which are directly opposite each other, are propelled, whilemaintaining this opposite positioning, at identical velocities andprecise spacings with adjacent, opposed, tenter clip pairs. Thisoperation is accomplished by use of synchronous linear motors.

More specifically, in the tenter frame apparatus and method of theinstant invention, two endless tracks guide individual active carriagesin loops that are arranged opposite one another with the film passingbetween the loops. Synchronous secondaries are attached to thesecarriages to which are also attached tenter clips to grip the edges ofthe film. Elongated primaries are located opposed to each other on thefilm drawing or forward side of each loop and are adjacent thesynchronous secondaries on the carriages to engage themelectromagnetically. In a significant aspect of the invention, eachprimary includes a plurality of groups of coils with the group of coilsin one primary sized to match the opposed groups of coils in the otherprimary and with each of the opposed groups of coils being electricallyjoined and defining a single control zone. Power is applied to thesecontrol zones to propel opposed pairs of carriages in symmetry througheach control zone and from one control zone to the next throughout thetenter frame. The tracks in each loop can diverge as in a conventionaltenter frame and simultaneously the carriages gripping the edges of thefilm can be separated in the MD and TD as the carriages travel along thetrack. The film is thereby simultaneously biaxially drawn or stretched.

The synchronous secondaries assure that the active carriages willsynchronously engage, or lock onto, the electromagnetic wave developedby the primaries resulting from control instructions. Therefore, as longas the carriages are continuously fed to the forward sides of the loops,preferably in stacks being propelled at controlled speeds, and theopposed groups of coils in each control zone simultaneously receivealternating current developed from the same predetermined controlinstruction, which are simultaneously coordinated with adjacent controlzones instructions, the carriages will remain in symmetry as they arecontinuously propelled along the forward sides of both loops.

The art mentioned above does not show or suggest the method justdescribed.

The Habozit patent, and the French patent, which is related to it, onlyshow an endless loop linear motor system where individually controlledfield windings along opposed loops accelerate carriages containing filmclips through a tenter oven. The induction secondaries are attached toeach carriage to propel them throughout the loops. There is no teachingas to how to achieve symmetry of motion of opposed clips, nor is thereany teaching of how to solve the unique problems of controlling aplurality of carriages having synchronous secondaries attached.

The Japanese patent publication 48-38779 only shows an apparatus topropel tenter clips in opposed loops for stretching film using a "shortcore-type linear motor". No means is provided to insure symmetry ofmotion of opposed pairs of clips.

The instant invention, by providing this controlled symmetry of motion,offers improvements over the tenter frame art.

This invention further involves the use of linear motors to propelcarriages in endless loops from the exit of carriage collectionsections, where they move abutted in stacks at a constant synchronousspeed, to a second or greater speeds to space the carriages apart, afterwhich such carriages are propelled back into the stacks prior toreaching the entrance of the collection sections. The speed at which thecarriages move after contact with the stacks is determined by the speedof the abutted carriages in the carriage collection sections, againstwhich the carriages newly arrived in the stacks are constantly pressing.The carriages move asynchronously after contact with the stacks andprior to reaching the entrance of the collection sections.

The movement of the active carriages into the stacks is controlled byhysteresis secondaries also attached to the carriages. These secondariesare engaged by electromagnetic waves independently developed by zones oflinear motor primaries positioned adjacent the hysteresis secondaries.The electromagnetic waves may vary in speed to control acceleration anddeceleration of the carriages.

Further, in another aspect of this invention, one or more passive oridler, clips may be placed between each of the active clips, in eachloop of the tenter frame, to minimize film edge scalloping as shown, forexample, in previously mentioned French patent 2,317,076. The carriagesto which these passive clips are attached are unpowered during the filmstretching operation. Such passive carriages are initially propelled byabutment with the active carriages having the synchronous secondariesattached thereto. As the active carriages separate to stretch the film,the passive clips are then propelled, on the forward sides of the tenterframe, by their engagement with the moving film.

After the film is released, these carriages are then returned, alongwith the active carriages, along the return sides of the loops and intothe stacks of carriages, prior to movement back into the film processingsection of the tenter frame. This is accomplished by attachinghysteresis secondaries to the passive carriages. These secondaries areadjacent the same primaries engaging the hysteresis secondaries on theactive carriages. The electromagnetic waves developed by these primariesact to propel the carriages along the return sides and into the stacksbefore they reach those parts of the stacks being propelled at constantcontrolled speeds, in the carriage collection sections adjacent theentrance to the tenter frame. This use of hysteresis linear motors topropel the carriages on the return sides of the loops and into thestacks of carriages is an important feature of this invention.

Linear motors, of course, are known to the art and such motors can beused to propel carriages or tracked vehicles in an endless loop.

U.S. Pat. No. 3,893,466 to Starkey is one example of such a teaching,showing the use of a linear synchronous motor propulsion system forindependently propelling tracked vehicles in a loop. The vehicles orincoming trains are selectively accelerated and then decelerated at anappropriate location to allow them to travel at a slow speed and at aselected spacing through a station.

U.S. Pat. No. 3,890,421 to Habozit is another example showing the use ofa linear induction motor for controlling the speed of clamps mounted oncarriages moving in endless loops for biaxially drawing plastic film.And Japanese patent application 48-38779 is still another exampleshowing the use of a linear motor to propel tenter clips in endlessloops to biaxially stretch thermoplastic synthetic resin films. There isno indication, however, as to how the carriages are controlled on thereturn sides of the loops.

It further is common practice, in systems involving carriages orvehicles traveling in an endless loop, to provide a loading station orstartup section where the carriages are moved at low speeds for loadingof passengers as in U.S. Pat. No. 3,803,466 or for other operations andare then accelerated in an operational section. In the startup sectionthe carriages are closely spaced and frequently are clustered or groupedin a stack as shown in the above-mentioned patents, for example. Afteracceleration or completion of the operation, such as film stretching,the carriages are returned to the stack, or loading section, ready tostart the operation again.

It is generally required that the carriages be under control at alltimes in their movement in and, through the loop. This is particularlytrue when the carriages or tracked vehicles, which are frequently movingat high speeds, are returned to the stack, otherwise damaging collisionsmay occur or machine operation may be affected.

Various techniques have evolved to solve this type of problem. One suchsolution is seen in U.S. Pat. No. 4,675,582 to Hommes and Keegan, ownedby the assignee of the present invention. This patent, which isincorporated herein in its entirety by reference, discloses a linearsynchronous motor control system which can be used to precisely propelsynchronous secondaries attached to carriages at ever increasing speedson the forward sides of a pair of opposed loops to stretch film, forexample. This same system also can be used to decelerate the carriagesunder control on the return sides of such loops. In such a system wherecarriage speed and spacing is varying, there can never be more than onecarriage in an electrically separate group of coil windings, or zone, ofthe primary at a time. This constraint requires many primary zones andtheir associated zone controls. Such a system effectively accomplishesthe task of continuously propelling carriages throughout an endlessloop, but at a significant cost in hardware and complexity, particularlyon the return side where stacking occurs and where such precisesynchronous control of the carriages may not be required.

In carriage or tracked vehicle propulsion systems the location and speedof the carriages at startup is frequently of prime importance. Forexample, the tracked vehicles in the Starkey patent mentioned above,appropriately travel at a selected spacing in the station, for loadingpurposes. This is true in other systems as well, including the systemshown in the Hommes and Keegan patent just described, where control ofthe secondaries, in a constant velocity section, with their preciselocations known prior to acceleration, is important in the operation ofthe system.

The instant invention, by assuring that the stacks of carriages movesynchronously and abutted at controlled constant speeds in carriagecollection sections, further assures that the carriages at startup willbe in the proper position and that they will be moving at a propercontrolled speed. In so doing, such invention provides an improved oralternate method for practicing the inventions of U.S. Pat. No.3,803,466 and of U.S. Pat. No. 4,675,582, particularly on the returnside, for example.

The location and speed of the carriages at startup is also of primeimportance for drawing film in a tenter frame. For example, instretching a web of film, it is important that the carriages to whichthe tenter clips are attached enter the tenter frame at a known spacing.This is true in other systems as well.

More specifically, in practicing the invention, the carriages must enterthe tenter frame in synchronism with the electromagnetic wave in thefirst control zone. Prior to machine startup the carriages are pressedup against one another with the carriage bodies abutted and the leadcarriage held stationary. This establishes the spacing at a knownunvarying value at which the magnetic pole pitch of the carriagesecondaries matches the electromagnetic wave pole pitch determined bythe coils in the primary.

After this pre-startup orientation of carriages is established, and theremainder of the forward sides of the loops are empty of carriages, thetenter frame can be started up and the carriages will be propelled oneafter the other along the forward sides in synchronism with theelectromagnetic waves and returned along the return sides. If theforward sides of the tenter frame are stopped in a controlled fashion,the relative positions of the carriages can be maintained and restartingdoes not require realigning of the carriages.

The instant invention by assuring that the stacks of carriages are movedsynchronously and abutted in the carriage collection sections furtherassures that the carriages at startup will be in the proper position andthat they will be moving at a proper controlled speed.

The tenter system of the invention also has means to alter the MD drawratio while continuing to simultaneously biaxially draw the film. Thispermits threading-up film at low MD draw ratios and then graduallychanging the MD simultaneous biaxial draw ratio to a higher level forcontinuous operation. The stack forming sections on the return sides ofeach loop can also gradually change the deceleration rate and stacklength to accommodate the shift in numbers of carriages from the filmprocessing sections to the stack forming sections as the MD draw ratiosincrease; which increase is also generally accompanied by aproportionate increase in peak carriage velocity that requires changesin deceleration rates. This unique feature to readily and rapidly changedraw ratios also permits rapid, low cost optimization of film drawingratios without having to shut down the line and fabricate and installnew parts for new incrementally changed draw ratios. In commercialsimultaneous biaxial film tenters, the simultaneous MD draw cannot bechanged after start-up, so the simultaneous MD draw ratio at start-upand the simultaneous MD draw ratio for continuous operation have to bethe same. For certain film polymers, however, there is the problem thatfilm tearing occurs when threading-up at high draw ratios. This problemis overcome by the system of this invention. Furthermore, such inventionoffers precise predictable control of carriage motion with few movingparts and an open-loop (no feedback) control system, and withoutfixed-pitch mechanical screws and chains, or position and drive signalfeedback systems. The instant simultaneous biaxial tenter frame canoperate at much higher draw ratios and line speeds than previouslypossible.

Accordingly, this invention makes available to the art improved methodsand apparatus for propelling carriages around loops in a tenter frame,and solves various problems heretofore confronting the art by providingan effective film drawing operation controlled by snychronous motors andby assuring that the carriages are moved back into stacks of carriages,in a controlled manner, using relatively inexpensive hysteresis motors.Such invention further assures that the carriages in those portions ofthe stacks in the carriage collection sections, are always abutted andthat they too, are moving at a proper controlled constant speed, priorto entering the forward sides of the tenter frame.

Such invention represents a major advance in the art not only of linearmotor systems, but of known systems for biaxially drawing plastic filmsin tenter frames.

SUMMARY OF THE INVENTION

Briefly described, the present invention provides an improved method andapparatus for drawing film in a tenter frame, in which synchronous andhysteresis motors are used to propel the tenter clips, under totalcontrol, throughout the opposed loops of such tenter frame.

BRIEF DESCRIPTION OF THE DRAWING

FIG. 1 is an isometric view of a linear motor propulsion system of thisinvention for propelling carriages or tracked vehicles around an endlessloop or track.

FIG. 2 is an isometric view of a dual-secondary carriage of theinvention.

FIG. 3 is a graph of one case of secondary (carriage) velocity versusposition as it travels around the endless loop.

FIG. 4 is an exemplary plan view of the system at a given instant intime showing the carriages distributed around the endless loop.

FIG. 5 is a plan view of an upper first primary for propelling thecarriages around a part of the loop.

FIG. 6 is a plan view of a lower second primary for propelling thecarriages around the rest of the loop.

FIG. 7 is a graph of force versus slip for a linear hysteresis motor.

FIG. 8 is a graph of three cases of secondary (carriage) velocity versusposition as it travels around the endless loop.

FIG. 9 is a graph of two stable operating curves for the stack formingsection showing the effects of changing friction on the carriages.

FIG. 10 is a schematic top plan view of a simultaneous biaxial tenterframe of the invention.

FIG. 11 is a typical cross-section of the two opposed loops taken alongline 11--11 in FIG. 10.

FIG. 12 is typical enlarged cross-section of the active carriageadjacent the elongated primaries in the film processing section seen inview 12 in FIG. 11.

FIG. 13 is a typical enlarged cross-section of the active carriageadjacent the elongated primaries in the stack forming section seen inview 13 in FIG. 11.

FIG. 14 is a top plan view of the active and passive carriages abuttedand gripping the film in the transport section.

FIG. 15 is a top plan view of the active and passive carriages separatedand gripping the film at the end of the drawing section.

FIG. 16 is an elevation view of an active and passive carriage seen inview 16--16 in FIG. 15.

FIG. 17 is a cross-section of the friction wheel engaging the carriagesat the end of the film processing section taken along line 17--17 inFIG. 10.

FIG. 18 is block diagram of a representative portion of the controlsystem of the invention and is based on FIG. 1 from U.S. Pat. No.4,675,582.

FIG. 19 is a graph of three cases of active and passive carriage(secondary) speed versus position as it travels around an endless loopof a simplified tenter.

FIG. 20 is a detailed schematic of a control zone driver shown in FIG.18 for a typical control zone in the carriage collection and operationalsections of the loops and is based on FIG. 8 from U.S. Pat. No.4,675,582.

FIG. 21 is plan view of an inflection point in the elongated primary onthe operational section of a loop.

FIG. 22 is a cross-section view of the inflection point of FIG. 21.

FIG. 23 is a detailed view of the control zone drive current control andgating logic shown in FIG. 20.

FIG. 24 is a representative plot of current versus time showing how theswitching rate of the control zone driver transistors is limited by thelogic of FIG. 23.

FIG. 25 is a diagram of the simplified tenter loop for the plots of FIG.19.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

In a linear motor propulsion system for propelling carriages around anendless loop, there are various ways of accelerating and separating thecarriages, but a simple reliable method and apparatus to decelerate thecarriages to rejoin a moving stack, in a controlled manner, has notheretofore been available.

The system of this invention provides such a method by continuouslypropelling spaced apart. carriages into a stack by controlling a linearmotor primary positioned adjacent the carriages to develop anelectromagnetic wave that acts on a hysteresis secondary attached toeach carriage. The primary is electrically divided into groups of coilwindings or zones that are each independently controlled. These zonescan be powered to propel the hysteresis secondary either synchronouslyor asynchronously from a second speed to a third speed just above thefirst speed of the stack. When the carriage encounters the stack, thezone at the stack entrance propels the hysteresis secondaryasynchronously to press the carriages together. This fixes the locationof the carriages at a known value so that subsequent accurate control ofcarriage movement and spacing is made possible.

The instant invention, in its preferred form, uses a combination ofhysteresis and synchronous secondaries to propel the carriages aroundthe loop and through the stack. The system provides continuouslypredictable control of the carriages on an operational side of the loopwhere carriage spacing and speed are controlled precisely and on areturn side of the loop where controlled abutting of the carriages inthe stack is accomplished. The linear motor controls for the operationaland return sides are coordinated so that as the spacing requirements arealtered on the operational side, the changes in the number of carriageson the return side can be accommodated without adding or removingcarriages from the loop. The controls for the two sides also coordinatespeed scale-up on the operational side and the resultant changes inbraking and stacking requirements on the return side.

The movement of the carriages is carefully and constantly controlled onthe operational side of the loop by the synchronous secondaries, andcarriage movement into the stack is controlled by the hysteresissecondaries. This novel use of dual secondaries on each carriage plays asignificant role in the operation of the preferred embodiment of thesystem.

Secondaries

In this application, a "synchronous secondary" is one which has apermanent magnetic pole or poles that exist in a magnetic field andremain the same regardless of the electromagnetic field of the motorprimary acting on them and a "hysteresis secondary" is one which has atemporary magnetic pole or poles that exist in a magnetic field andremain the same unless the electromagnetic field, such as that of themotor primary, changes polarity so it is not aligned with the hysteresissecondary. When in a strong non-aligned field, the hysteresis secondarypolarity changes to correspond (opposing polarity) to the polarity ofthe new electromagnetic field.

A synchronous secondary can only develop its rated force when it ismoving synchronously at the same speed, i.e., no slip, as the travelingelectromagnetic wave (hereinafter frequently referred to as an "EMWave") propelling it, and when its polarity is properly aligned with thewave. When slip occurs, the synchronous secondary force and speed becomeerratic and the secondary may come to a stop.

A hysteresis secondary can develop at least a first force when it isoperating essentially synchronously (no or very low slip) or a secondforce when it is operating asynchronously (substantial slip) with thetraveling EM wave. In the region of very low slip, the force is betweenthe level of no slip and substantial slip. The hysteresis secondary canbe propelled essentially synchronously with the EM wave as long as anopposing force does not exceed its second force so it can travel at ornear the same speed as the EM wave. It can also be propelledasynchronously traveling at a speed substantially different from the EMwave and continue to develop its second force regardless of the amountof slip. When its second force is greater than the opposing force, thehysteresis secondary speed will essentially reach that of the EM wave.When its first force is greater than the opposing force, the hysteresissecondary speed will reach that of the EM wave and its polarity willcorrespond (opposite poles) to that of the EM wave, and the secondarywill travel synchronously with it.

To summarize, a synchronous secondary can only be propelledsynchronously with an EM wave, while a hysteresis secondary can bepropelled either essentially synchronously or asynchronously with suchwave. In the instant invention, this latter principle enables thehysteresis secondaries attached to the carriages to constantly presssuch carriages together after they contact a stack and before they reachthe entrance to a carriage collection section, where their movement iscontrolled prior to operational startup of the system of such invention.

Endless Loop

Referring specifically to the drawing, FIG. 1 shows the endless looptraveled by the carriages, as propelled in accordance with thisinvention. Such carriages, 1, are supported and guided by a track 2 thatdefines the path the carriages take around the loop. Upper and lowersecondaries 3 and 4 are attached to each carriage body. The trackpositions these secondaries 3 and 4 fixed distances (exaggerated forclarity) away from upper and lower linear motor primaries 5 and 6, whichare positioned adjacent the track.

As best seen in FIGS. 1 and 2, each carriage 1 is generally a "C" shapedstructure that largely surrounds the track 2. On the outside of the "C"shape is a first surface 7 suitable, for example, for mounting anarticle that is to be moved by the carriage.

In a preferred embodiment, a synchronous linear motor secondary 3 isattached to a second surface 8 at the top of the "C" shaped carriage 1.Such secondary includes two permanent magnets 9 and 10 with alternatepoles facing outwardly to complete a magnetic flux path, shown at 11,which includes the upper primary 5. The magnets which are affixed to ahigh magnetic permeability back iron material 12 such as steel or castiron, are spaced with their poles apart at a distance, lambda s, equalto the pole pitch of the upper primary 5 and they form a third surface85 which is compatible with the shape of the opposing surface of theprimary 5. The magnet material may be conventional permanent magnetmaterial such as tungsten or chrome magnet steel, or permanentrare-earth magnets such as aluminum-nickel-cobalt alloys (alnico),cobalt magnet steel, or preferably samarium cobalt.

Further, in this embodiment, a hysteresis linear motor secondary 4 isattached to a fourth surface 13 at the bottom of the "C" shapedcarriage 1. This secondary consists of hysteresis material 15, whichforms poles on its lower face when in the electromagnetic field producedby the lower primary 6. The secondary may also preferably include a highmagnetic permeability back iron 14. In some cases, however, it may bemore convenient to mount the hysteresis material on a surface having alow permeability such as aluminum, or a non-metal surface, or to mountthe hysteresis material along its edges with no backing surface. It mayalso sometimes be desirable to mount the hysteresis material on agrooved, high permeability surface. Such variations are often used inrotary hysteresis motors. The lower face of the hysteresis materialforms a fifth surface 86 which is compatible with the shape of theopposing surface of the lower primary. The carriage body 84 may be madeof a high magnetic permeability iron or steel that conveniently alsoforms the back irons 12 and 14. The geometry (thickness, shape, area) ofthe hysteresis material is a factor determining the force developed inthe EM field. The hysteresis material is one having a high magnetichysteresis and may be unmagnetized magnet material as listed above, andis preferably unmagnetized alnico. A flux path similar to that shown at11 is formed with the lower primary 6 and the hysteresis secondary 4.The novel carriage 1 with its dual secondaries 3 and 4 is significant inthe preferred operation of this invention.

The pole pitch of lower primary 6, lambda h, does not have to match thepole pitch lambda s of upper primary 5 and, since the hysteresismaterial does not have fixed poles, the pole pitch of the lower primarycan be any convenient pitch. In a preferred embodiment, however, thepole pitch of the upper and lower primaries is the same, i.e., lambdas=lambda h=lambda.

A system of eight rollers 16, on each carriage ride on four elongatedsurfaces of the rectangular track 2 to maintain the orientation of thecarriage, as shown, and provide anti-friction travel of the carriagealong the track. The outer surfaces of the magnets 9 and 10 and thehysteresis material 15 are closely spaced by constant distance clearancegaps 82 and 83 (exaggerated for clarity) with the adjacent surfaces ofthe primaries 5 and 6, as shown in FIGS. 1 and 2. The surfaces of theprimaries and the magnets and hysteresis material can be planar, asshown, or for certain applications the primaries may have an elongatedconcave or convex surface with the magnets and hysteresis materialshaped to be compatible. For instance, the magnets could define a convexsurface, elongated in the direction of travel, which is closely spacedfrom an elongated concave primary surface. If the hysteresis secondariesare always traveling along a radius when adjacent the lower primary, theouter surfaces could also be a toroidal surface. In general, the outersurfaces of the secondaries and the compatible surface of the primariesare shaped to maintain a constant clearance gap between the movingsecondaries surface and the closely spaced surface of the primaries.

As best seen in FIGS. 1, 4, 5 and 6, the upper or first primary 5 islocated adjacent the track 2, in part of the loop, as shown at 17, inposition to act on the upper synchronous secondary 3 on each carriage.The lower or second primary 6 is located in another part of the loop, asshown at 18, adjacent the track 2, to thereby act on the lowerhysteresis secondary 4 on each carriage. These primaries propel the dualsecondaries, and the carriage to which they are attached, in an endlessloop, through the three prime functional sections of the system; acarriage collection section 41, an operational section 42 and a stackforming section 43. The operations in these sections will be describedlater in greater detail.

FIG. 4 is a plan view of a "snapshot" of the carriages 1, indicated bycircles, traveling around the endless loop. The carriages are shownsuperimposed on the upper and lower primaries 5 and 6. FIGS. 5 and 6 areplan views of such primaries (shown shaded) showing their relativepositions, 17 and 18, in the loop.

Upper Primary

The upper linear motor primary 5 is divided into a plurality of zones,such as first zone 19 and second zones 20, 21, etc., 22, as shown inFIG. 5, to permit synchronous control of the synchronous secondaries 3in their movement in the carriage collection and operational sections 41and 42 of the loop. A suitable system for controlling such secondariesis shown in U.S. Pat. No. 4,675,582 to Hommes and Keegan, mentionedpreviously.

In this patent, a synchronous linear motor primary is electricallydivided into groups of coil windings or zones with each zone beingindependently powered and controlled. Each zone is powered by a zonedriver having switches to switch a DC voltage to provide a synthesizedthree phase AC power waveform to the coils in each zone. Each zonedriver has a zone controller with a steady state memory and memoryaccess means to provide switching instructions to the zone driver for agiven operating condition. The memories in each zone controller containthe same number of predetermined binary switching instructions to definea switching waveform that can slope in frequency between two limits. Thesteady state switching instructions are simultaneously output from allzone controllers and are paced by a common time base to start, stepthrough, end, and restart the accessing of each memory in each zonecontroller in unison. In an acceleration portion 57 of the operationalsection 42 of the system of the instant invention, this causes varyingspeed, repeating EM waves to be developed along the primary that arecoordinated in time to independently proper one synchronous secondaryafter another through the acceleration portion. A plot of frequency vs.time of such a wave would have a sawtooth shape. In this portion thereis never more than one secondary in a zone at a time. When it is desiredto change operating conditions, a transition memory in each zonecontroller is accessed that has instructions to propel severalsecondaries, each uniquely as required. At the end of the transition, adifferent steady state memory is accessed repeatedly to propel eachsecondary the same for as long as it is desired to remain at the newoperating condition. A central controller coordinates the simultaneoustransfer from one control memory to another in all zones. A systemcomputer coordinates overall system operation.

The carriage collection and operational sections 41 and 42 of theinstant system are closely coordinated and carriage movement in suchsections is under the control of the upper first primary 5, in this partof the loop. In these sections each zone of the primary, including firstzone 19, and second zones 20 through 22, has its own independent driverand controls, such as 23, 24, etc. and 25, that are electricallyconnected respectively to such zones and are coordinated by a centralcontroller 27, which includes a time base 40, and a computer 51 todefine a linear synchronous motor control system. This system providessynchronous control of a plurality of carriages in part of an abuttedstack of carriages in the carriage collection section 41 as powered byzone 19, as controlled by driver and control 23. This system alsoprovides independent synchronous control of each carriage 1 as itaccelerates and separates from adjacent carriages along the operationalsection 42 of the loop. Although there are plurality of zones shown inthe acceleration portion 57 of the operational section, in a simplestcase, where the synchronous secondaries on adjacent carriages are widelyseparated when the carriages are abutted, the acceleration portion 57may consist of only a single zone several lambda long. With this initialwide spacing, the secondaries can be independently accelerated and neverhave more than one secondary in one acceleration zone at a time.

Lower Primary

While the synchronous linear motor system just described is highlyeffective for accelerating carriages, and further may be used to returnthe carriages to a stack, this invention offers an improved system,particularly for stacking the carriages on the return side of the loop.This stacking operation is under the control of the lower linear motorprimary 6.

Such linear motor primary is also divided into a plurality of zones,e.g., third zones 28, 29, 30 and 31, as shown in FIG. 6, to permit acontrolled, stepped deceleration of the carriages as they pass throughthe stack forming section 43, of the loop, and cause controlled impactand pressing abutment of the carriages before they meet the accumulatedstack of carriages in the carriage collection section 41. In each lowerprimary zone the speed of the EM wave is constant, not varying, andsince a synchronous relationship with the hysteresis secondary on thecarriage does not always exist, there is no critical phase relationshipbetween zones that requires precise waveform coordination between suchzones. For these reasons, conventional motor drives and controls can beused for each zone of lower primary 6, such driver/controls 58, 59, 60,61 that are electrically connected respectively to the third zones 28,29, 30 and 31 of the primary. Conventional inverter type drives arepreferably used in this part of the system. Their frequency iscontrolled by the computer 51, based on system requirements.

The carriage collection section 41 has an entrance and an exit, as shownin FIG. 4. It is critical, in the operation of the system of thisinvention, that the carriages 1, under the control of the hysteresissecondaries 4, be propelled into the moving stack of carriages prior toreaching the entrance to the collection section. Specifically, suchcarriages are propelled by zone 31 of the lower primary 6 in thisportion of the stack forming section 43, which zone acts on eachhysteresis secondary 4 to propel the carriage into the stack and tocontinue that pressure so to push the carriages together, prior toreaching the entrance of the collection section 41.

Carriage Stack

It is important, during system operation, that there is a stack ofmoving carriages 80 in the loop, such as the one shown in FIG. 4. Suchstack has an entrance at about 34 and an exit at about 33. By "movingqueue" or "moving stack" of carriages is meant a region of abuttedcarriages that has the exit end of the region "fixed" in position in theloop, and has moving carriages continuously entering the entrance end,moving through the stack, and leaving the exit end. At this exit end ofthe stack, the carriages should be propelled "synchronously" at aprecisely known speed and position determined by some means engaging thecarriage such as a sprocket, screw thread, or synchronously actinglinear motor. This ensures that when it is desired to separate thecarriages, their exact position and speed are already precisley knownwithout the need for sensors or other feedback. The stack 80 itself doesnot travel around the loop but one end of the stack may move as thequantity of carriages in the stack change. The carriages within thestack are all traveling at the same speed as they move through thestack. Within the stack, the spacing between carriages is constant andthe carriages are preferably abutted. The position in the loop of theentrance and exit ends of the stack, as distinguished from the entranceand exit of the carriage collection section 41, varies by at least thewidth of a carriage as a carriage instantaneously abuts or separatesfrom the stack and thereby redefines the position of the stack ends.

The abutted stack of carriages 80 must always completely fill thecarriage collection section 41, in which synchronous propelling of partof the stack is required. As shown in FIG. 4, the carriage collectionsection is defined by the first synchronous zone 19 that engages aplurality of synchronous secondaries 3 on the carriages that are abuttedto progressively develop a propelling force that exceeds any otherforces on the carriages in the stack. Such other forces may be a stackpressing force (developed in the stack forming section to be explainedlater), plus frictional forces on the carriages and external forcesplaced on the carriages in the stack, such as film tension, for example,where the instant system is used in a film tenter operation. If all ofthese forces on the carriages in the stack are low, the carriagecollection section could be shorter than shown, for instance it may onlyextend from position 69 and 33 in FIG. 4, where the carriages are shownsynchronously propelled in the stack by zones 20 and 21 in theoperational section. It is preferred, however, to provide a separatezone, zone 19, to provide synchronous propelling of part of the stack.The secondaries 3 on the carriages must always enter zone 19 at a fixedspacing and in synchronism with the EM wave developed there.

Abutting of the carriages is the best way to precisely fix secondary tosecondary spacing in the stack forming section of the loop, where thehysteresis secondaries 4 can slip varying amounts on the EM wave, butwithin predictable limits. The trailing carriage in the stack mustalways be located before the end of lower primary 6 since the forcedeveloped on the hysteresis secondaries determines the total pressingforce keeping the carriages abutted before entering the carriagecollection section 41. The end of the lower primary is fixed in the loopby end 74 of zone 31. The stack entrance, at 34, must also be locatedafter a position, at about 52, where the carriages have decelerated toan "overspeed" slightly greater than the stack speed. This position isnot fixed in the loop but varies with a given operating condition, aswill be best understood during the discussion of FIGS. 8 and 9. Theoverspeed is predetermined by the preset EM wave speed in hysteresiszone 31 at the end of the stack forming section. This velocity may befrom about 5-100 feet per minute higher than the stack speed determinedby the EM wave in synchronous zone 19. The impact speed is defined bythe difference between the overspeed and the stack speed and must bekept low to avoid damage to the carriages entering the stack. Theoverspeed, determined by the EM wave engaging a hysteresis secondary,acts to propel the carriages into controlled abutment with the entranceend of the stack and propels the carriage into pressure abutment withinthe stack.

The minimum deceleration distance of each carriage is determined by theinitial carriage speed, the total weight of the carriage, frictionalloads on the carriage, and the force developed by the EM wave on thehysteresis secondary (a function of coil current, magnetic air gap,slip, and secondary geometry). These factors must be taken intoconsideration in determining the maximum allowable entrance end-of-stackposition on the stack forming side of the loop. In most situations thereis an effort to keep all of these factors constant, but mechanicaltolerances cause slight variations from carriage to carriage. The randomnature of these variations, however, will average to a "constant" valueover many carriages.

The entrance end of the abutted stack of carriages occurs before thecarriages leave the stack forming section and enter the carriagecollection section at 32. The speed of the stack, however, is set by thespeed of the EM wave, in zone 19, acting on the synchronous secondaries3, where no slip occurs, and the stack is being propelled at a constantknown speed. The pole pitch of the EM wave in zone 19 equals the polepitch of the stacked carriages so all the carriages in this zone can bepropelled simultaneously by a common EM wave. From the end of the stackto the end of the lower primary zone 31, from 34 and 74, however, the EMwave in zone 31 in the stack forming section is developing a force onthe hysteresis secondary 4 on each carriage to press the carriages intoan abutted condition. This hysteresis secondary stack force must alwaysbe less than the total pull-out force of the synchronous secondaries inthe stack, so synchronous propelling of the stack is maintained. Fordifferent operating conditions where the stack length increases and morehysteresis secondaries are being propelled in the stack, the forcedeveloped by each of the hysteresis secondaries may have to be decreasedto maintain the total hysteresis stack force below the total synchronouspull-out stack force. It is also desirable to keep the total hysteresisstack force low to avoid unduly high contact forces between thecarriages.

System Summary

In summary, then, this invention, briefly described, includes a systemfor propelling carriages from a stack in a carriage collection section41 to a spaced-apart condition and back to the collection section bypropelling the carriages along an endless track 2, using a linear motor.Such system, in a preferred embodiment 1 includes a first primary 5positioned along a first part of the track and a second primary 6positioned along a second part of the track. The carriages, which areguided around the track, each has a synchronous secondary 3 positionedadjacent the first primary 5 and a hysteresis secondary 4 positionedadjacent the second primary 6; control means are provided for eachprimary whereby the first primary 5 propels the carriages through thecollection section 41 and accelerates them from an abutted to aspaced-apart condition, and whereby the second primary 6 decelerates thecarriages, propels them into an abutted condition in a moving stack andapplies pressure to the abutted carriages before the collection sectionentrance.

The entrance to the carriage collection section 41 is located at thebeginning of the first primary 5. The trailing carriage in the movingstack of abutted carriages is always located before the end of thesecond primary 6.

Further, in this embodiment, the first primary 5, is provided with coilselectrically grouped into zones, including a first zone 19 and aplurality of second zones 20-22. The second primary 6 also has its coilselectrically grouped into a plurality of third zones 28-31.

The system's basic operational units include a carriage collectionsection 41, an operational section 42 and a stack forming section 43.The carriage collection section has an entrance and an exit and one end32 of the first zone 19 of the first primary 5 is located adjacent theentrance to the carriage collection section 41 and the other end 69 ispreferably located adjacent the exit of such carriage collectionsection. Further, in this embodiment, one end of the first of theplurality of second zones 20-22 of the first primary 5 is locatedadjacent the start of the operational section 42 and the other end ofthe last of the plurality of second zones of the first primary 5 islocated adjacent the finish of the operational section. The plurality ofthird zones 28-31 of the second primary 6 are located adjacent the stackforming section. The system has means to independently developelectromagnetic waves in each of the zones for controlling thepropelling of the carriages 1 through all the sections.

The system just described is adapted to propel carriages around a loopand into and through a stack by a novel method or methods of operation.In so doing such carriages are propelled, in the preferred embodiment,by a linear motor having hysteresis and synchronous secondaries 3 and 4attached to the carriages 1 that are traveling continuously in anendless loop defined by a guide track and are propelled by discretezones of linear motor primaries positioned adjacent the track. Suchmethod includes the steps of:

propelling the carriages along the first zone 19 of the first primary 5by acting on the synchronous secondaries 3 to propel the carriages in anabutting relationship in a stack, in a carriage collection section 41,at a first constant speed;

propelling the carriages along the second zone or zones 20-22 of thefirst primary 5 by acting on the synchronous secondaries 3, one by one,to accelerate the carriages, in an operational section 42, from thefirst speed abutted to a second speed spaced apart;

propelling the carriages along at least a third zone 31 of the secondprimary 6 by acting on the hysteresis secondaries 4 to decelerate thecarriages, in a stack forming section 43, from the second speed spacedapart to a third speed close together, the third speed being greaterthan the first speed;

essentially synchronously propelling the carriages along one part of thethird zone 31 at the third speed prior to contact with the abuttedcarriages in the stack forming section 43; and

asynchronously propelling the carriages along another part of the thirdzone 31 to produce and abutting force which presses the carriages intothe abutted carriages which are moving at the first speed in the stackforming section 43.

This invention also includes a method of controlling the propulsion of aplurality of spaced apart carriages into a stack of moving abuttedcarriages including the steps of propelling the abutted stack ofcarriages at a first speed and propelling a spaced apart carriage into acontrolled impacting abutment with the stack of abutted carriages bypropelling the spaced apart carriages at a third speed greater than thefirst speed before the carriage reaches the trailing carriage in thestack, using a linear motor.

This invention also includes a method of controlling the propulsion of aplurality of spaced apart carriages into a stack of moving abuttedcarriages, using a linear motor, including the steps of propelling aspaced apart carriage essentially synchronously with respect to anelectromagnetic wave at a third speed, in a stack forming section 43,prior to reaching the trailing carriage in the stack which is travelingat a first speed less than the third speed, and propelling the carriagesasynchronously with respect to the same electromagnetic wave, in thestack forming section 43, after the carriages reach the stack. In thismethod, the linear motor comprises a primary and at least a hysteresissecondary attached to each of the carriages and the abutted carriages inthe stack move synchronously at the first speed with respect to anotherelectromagnetic wave.

The invention also includes a method of propelling carriagesasynchronously with respect to an electromagnetic wave in one part of astack and synchronously with respect to another electromagnetic wave inthe other part of the stack. The carriages are propelled asynchronouslywith respect to the EM wave developed by zone 31 of the lower primary 5and synchronously with respect to the EM wave developed by zone 19 ofthe upper primary 5.

The third zone 31 of the lower primary acts on the hysteresissecondaries 4 to propel the carriages into the stack, prior to theirentrance into the carriage collection section 41. In so doing, suchsecondaries and associated primary together function as a hysteresislinear motor in this significant part of the overall system.

Hysteresis Linear Motor

In greater detail, a hysteresis linear motor develops approximatelyconstant force over a large differential speed, i.e., slip, between thesecondary speed and the EM wave speed. As with any motor, the EM wavespeed is determined by the fixed pole pitch (lambda) of the primarywindings, and the frequency of the alternating current in the coils (EMspeed=2 lambda f). The force level developed by the hysteresis motorconfiguration is determined by the level of current in the coils in theprimary that develops the EM wave acting on the hysteresis secondaries.A representative graph of the linear hysteresis motor force versus slipis shown in FIG. 7 for a given coil current. The force level and slopeof the curve will vary with current and also changes slightly with slipalong the curve at 75 and 76, for example.

As can be seen in FIG. 7, the hysteresis force does not remain at itssecond force level 87 and 88 at zero slip, but rather it tapers offwithin about ±3 lambda/sec slip. The exact nature of the curve in thisregion is uncertain and is therefore not depicted in the figure. At zeroslip, or synchronous speed, the motor behaves similar to a permanentmagnet synchronous motor and the developed force is just enough tocounteract the loads on the motor. In this example, the presence of afriction force acting on the carriage is shown superimposed at 77 and 78for comparison. On the left of the plot, the hysteresis motor andfriction act in the same direction, therefore, they work together toslow the carriage and reduce slip. On the right hand side of the plot,they act in opposite directions; if the friction force is greater thanthe first motor force 81 at zero slip (as illustrated at 78), thefriction force will continue slowing the carriage until the carriage isgoing slower than the EM wave and positive slip exists. The system isnow operating on the right side of the plot. As the carriage continuesslowing, slip increases and the hysteresis motor force increases untilit equals the friction force as at 79. A force balance then occurs andthe slip stabilizes; friction forces now no longer decrease carriagespeed. The carriage speed is now stabilized or essentially locked ontothe EM wave speed at a low slip of about 1.4 lambda/sec as shown. Thisessentially locked-on condition does not necessarily occur when the EMwave speed and the carriage speed are exactly matched, but it doesrepeated occur at a predictable low slip. If the friction force is lowerthan the first motor force at 81, the carriage speed will exactly matchthe EM wave speed. At this stabilized, essentially locked-on conditionand especially when the carriage is traveling synchronously at exactlythe EM wave speed, the effect of random variables between carriages isessentially eliminated and all carriages travel at essentially the samespeed. This obviously is a desirable condition that prevents carriagecollisions.

To summarize the modes of operation possible with a hysteresis secondarylinear motor, it can operate as follows:

"exactly synchronously" or "synchronously"--this is possible whenexternally applied forces are less than the first levels at about 81 and89 in FIG. 7 and slip is zero;

"essentially synchronously"--possible when externally applied forces areless than the second levels at about 87 and 88 in FIG. 7 and slip ispredictably a low value and changes only slightly with load; and

"asynchronously"--possible when externally applied forces are greaterthan the second levels at about 87 and 88 in FIG. 7 and slip may changeconsiderably with load.

System Operation

When decelerating the carriages, the linear hysteresis motor generatesan opposing essentially constant force over a certain distance, therebyremoving kinetic energy from the carriage. The carriage energy isdetermined by its mass and velocity. Removing energy, therefore,decreases its velocity. The rate of deceleration can be varied by one oftwo methods. A first method is to vary the current to the primary,thereby varying the force developed on the hysteresis secondary and thecarriage. A second method is to keep the current, and therefore theforce, constant and vary the distance over which the force acts. Thissecond method is accomplished by varying the distance over which slipoccurs before the hysteresis secondary essentially locks onto the EMwave. Friction on the moving carriage also acts to decelerate it, butthis is usually a force that is difficult to control and it may varyfrom carriage to carriage. It is desirable, therefore, to minimize theeffects of slight variations in frictional force by operating thehysteresis motor at relatively high force levels so frictionalvariations have a small relative effect on the total deceleration forceacting on the carriage. For this reason, the second method of varyingthe deceleration is preferred, since the motor force can remain at anearly constant high level. This method will now be described in greaterdetail, referring specifically to FIG. 3.

A typical operation of the system of this invention is best seen byreferring to FIGS. 3, 4, 5 and 6. To simplify the discussion, it isassumed that the maximum achievable motor force at zero slip is higherthan the friction force so the carriage speed equals the EM wave speedand the carriage will travel exactly synchronously. In FIG. 3, the solidline plot of speed versus position represents the carriage speed and thedashed line plot represents the EM wave speed seen by a secondary, shownslightly offset for clarity. The origin and end positions on the plotsjoin to define the loop and roughly correspond to position 53 in FIG. 4.The carriages are propelled from a continuous abutted stack extendingfrom the position 33 in the operational section 42 of the loop to aspaced apart condition, at 35, and then are moved together along thestack forming section 43 of the loop and enter the stack at about 34.The carriages then move through the carriage collection section 41 in anabutted condition with their speed controlled by the EM wave developedin zone 19 acting on the upper synchronous secondary 3 of each carriage.The carriages are individually accelerated in the acceleration portion57 by the independently controlled upper primary zones 20-22 of theupper primary 5, which develops EM waves that act to individuallyaccelerate the upper synchronous secondaries 3. In the operationalsection extending from 69 to 35 there are thirteen independentlycontrolled zones, in the embodiment shown.

In the example shown in FIG. 4, there are thirty-six carriages in theendless loop. The carriages enter the acceleration portion 57 of theoperational section 43 abutted and spaced one carriage length betweencenters, traveling at a first speed of 22.2 lambda/sec. At the end ofthe acceleration portion they are shown spaced 4 carriage lengths apartand reach a final second speed of 88.8 lambda/sec before leaving theoperational section 42. The carriages have undergone a speed and spacingchange of 4 x. The frequency of the AC power developing the EM wave ischanging in a repeating sawtooth pattern in each zone to cause thisacceleration. The secondaries are not in a zone, however, during thatzone's reset portion of the sawtooth pattern. They only see acontinuously increasing EM wave, as depicted by the sloping part of thedashed plot in FIG. 3. In the zones within operational section 42 thereis never more than one carriage in a zone at a time.

Near the end of the operational section 42, the lower primary begins at70 and develops an EM wave that acts on the lower hysteresis secondary 4on each carriage. The last upper primary EM wave, developed in zone 22,at its maximum speed is traveling at 88.8 lambda/sec, as shown at 44 inFIG. 3, and is propelling the synchronous secondaries 3 at this speed aseach carriage approaches the end of such zone. The first lower primaryEM wave, from zone 28, is set to travel at a speed of about 76.9lambda/sec, at 45, determined by the constant frequency of the AC powerdeveloping the wave in the primary. The hysteresis secondary on thecarriage will initially slip on this wave and start decelerating as itenters the stack forming section, at 35. The carriage will continuedecelerating until it reaches the EM wave speed of zone 28, at about 46.There may be several carriages in a single deceleration zone, such aszone 28, at a time. Upon reaching the EM wave speed, the hysteresissecondary 4 will stop slipping and act like a synchronous secondary andwill cause the carriage to travel synchronously at the wave speed of76.9 lambda/sec.

The EM wave in the next lower primary zone is set to travel at a speedof 59.4 lambda/sec, at dashed line 47, which will act to furtherdecelerate the hysteresis secondary 4 attached to each carriage from76.9 lambda/sec to 59.4 lambda/sec. The next lower primary zone 30further slows the carriage to 33.9 lambda/sec, at 48, and the followingzone 31 brings the carriage down to the overspeed velocity, or thirdspeed, of 26.7 lambda/sec, at 49.

The carriage encounters the stack, which is traveling at the first speedof 22.2 lambda/sec within zone 31, at about position 34. This stackspeed is determined by the EM wave velocity of 22.2 lambda/sec at dashedline 50 in zone 19 in the carriage collection section 41. Thedifferential velocity, or impact speed, between the carriage and stackat impact is 4.5 lambda/sec which is sufficiently low that it can beabsorbed by conventional shock absorbing devices on each carriage, suchas a rubber bumper. After absorbing the impact, the shock absorbingdevice should not prevent the carriages from abutting at the properpitch to be in synchronism with a common EM wave propelling the multiplecarriages simultaneously, as in zone 19, in the carriage collectionsection 41. Since the secondaries are not independently accelerating inzone 19 they can all be synchronously driven by a common EM wave in thiszone.

For a given total number of carriages in the loop, the stack lengthchanges as a result of changes in the spacing of the carriages in theoperational section of the loop and the final velocity reached. If theoperational section carriage spacing decreases, the number of carriagesin that section increases and the number in the stack forming sectiondecreases. Conversely, if the carriage spacing in the operationalsection increases, the number of carriages in that section decreases andthe number in the stack forming section increases.

The distribution of the carriages in the stack forming section may vary,however, depending on the amount of energy that must be dissipated,based on the carriage final velocity, and the constraints of some"desired results" explained below. In some instances, where it may bedesirable to minimize the number of carriages in the loop, it ispossible to increase the carriage speed above the second speed as thecarriages leave the operational section and enter the stack formingsection. This would cause the carriages to reach the stack sooner thanthe preferred case described where no acceleration occurs in the stackforming section. For the same stack length, when the carriages get tothe stack sooner, fewer carriages are required in the stack formingsection and, therefore, in the system. The stack length or stackentrance position will be affected by the carriage distribution which isbest understood by referring to FIG. 8 to be discussed later.

Deceleration and Stacking

In selecting the deceleration profile, i.e., the speed versus positionof the carriage as it decelerates, there are usually many differentprofiles that will achieve the desired results. The critical parameterthat must be controlled is the position of the entrance end of thestack. Two critical conditions must be maintained:

First, the minimum, or shortest, stack entrance position "B" must occurwell before the carriages leave the last lower primary (hysteresis) zone31 as shown in FIG. 3; otherwise, the carriages will not be pressedtogether and may arrive out of sync with the EM wave in upper primaryzone 19, and subsequent control of the carriages in the synchronousacceleration portion is not possible.

Second, the maximum, or longest, stack entrance position must not occurbefore the carriages have reached the overspeed velocity such aspreceding position "0"; otherwise, the impact velocity is no longeraccurately controlled and small stack end position variations result inlarge changes in impact velocity which may eventually damage thecarriages.

These two conditions still leave the solution for selecting the preciseoperating conditions undetermined. Other constraints useful to select EMwave speed set-points for the return zones will be discussed. Indescribing the hysteresis motor above, the preferred method to controlthe hysteresis motor is to keep the force, i.e., current, constant andvary the distance over which the carriage is decelerated in each zone.Some "desired results" in decelerating the carriage at a constant forceare the following:

A. Each carriage should essentially lock onto the EM wave in eachdeceleration zone before leaving the zone, so the carriage speed ischecked to a known value in each zone;

B. Each carriage should reach the actual entrance end-of-stack position,S, at least some preselected distance, p (say 4 lambda), after reachingoverspeed (position "0") and at least some preselected distance, r (say10 lambda) before reaching the minimum end of stack position B. Thisallows for some variation in position S without reaching the limit ofthe two critical conditions.

C. The distance, d, each carriage travels at the overspeed velocitybefore reaching the actual end-of-stack position is minimized to lessthan some preselected distance, u (say 8 lambda). Small values for dcauses the carriage to catch up to the stack more rapidly. However,distance d should also be somewhat greater than p to avoid operatingnear a limit that would require changing several control zone frequencysettings for small changes in operating conditions. Distance d, thenshould be more than p and less than u (4 lambda <d <8 lambda; actualvalues will vary with the overall size of the system, number ofcarriages involved, speeds, etc.)

D. When changing from one operating condition to another, thedeceleration should always be altered progressively from one zone to thenext in a smooth systematic fashion.

E. The stack should be kept as long as possible within the limits of theother constraints. To accomplish this, the carriage speeds should bekept as high as possible in all zones in the stack forming section.Having a long stack permits the greatest drift in stack length duringactual operation before the first critical condition is violated whichresults in misoperation of the system.

This last "desired result" is an important one that has significantadvantages that enhance reliable operation of the system. The carriagetravel is most predictable when traveling essentially locked onto theconstant speed EM wave. By keeping the speed high and constant for aslong as possible in the stack forming section, and then deceleratingrapidly in a short distance, the carriages reach the stack quickly whichkeeps the stack long. At the high constant speeds there is minimalchance for collision; within the stack, obviously, there are nocollisions; and the rapid deceleration exposes the carriage to collisionfor the shortest possible time. Further, the high motor force requiredfor rapid deceleration tends to minimize some of the random effects,such as friction.

The EM wave velocities of the deceleration zones, determined by theirdrive frequencies, can be selected using any of a number of controlalgorithms. One such control algorithm uses a set of iterations basedupon the percent of energy dissipated in each zone. This percent energydissipated in a zone is equal to the energy used to decelerate acarriage in that zone given by the average decelerating force, F, timesthe distance, delta-L, over which the deceleration is occurring; dividedby the total possible energy that can be provided by a zone which isgiven by the force, F, times the length of a zone, L. The algorithmrequires that the same percent energy be dissipated in each zone inwhich a carriage is decelerated. During an iteration, a potential set ofdeceleration zone speeds are selected. These speeds are then enteredinto a kinematic model that calculates the motion of the carriagethrough the system based on known deceleration forces such as from FIG.7. The initial set of iterations determines where (which zone) thedeceleration should begin. The final set of iterations adjusts thepercent energy dissipated in each deceleration zone until the resultsare in compliance with all the constraints, such as the "desiredresults" mentioned above.

System Stability

FIG. 8 shows three plots 37, 38 and 39 of speed versus position in theloop, similar to FIG. 3, but three different speed ratios are shown toillustrate how the system compensates and the actual entranceend-of-stack position can shift. Curve 37 shows a first operatingcondition that achieves a 4:1 velocity ratio, as in FIG. 3, but at lowervelocities. Notice the entrance end-of-stack, S, in FIG. 3, is at 135lambda while in FIG. 4, S-37 is at 121 lambda making the stack longer.The number of carriages in the stack forming section, however, is thesame in both cases, but the carriage energy (1/2 Mv²) in FIG. 3 wherethe carriage decelerates from 88.8 lambda/sec is much greater than inFIG. 8 where the carriage decelerates from 44.4 lambda/sec. Since it ispreferred that the deceleration force be the same in both cases, in FIG.3 the force must be applied over a much longer distance to dissipate thegreater energy. The length and number of deceleration zones also effectsthe distribution of secondaries since the carriage cannot startdecelerating in the middle of a deceleration zone. The deceleration"step" can only be taken starting at the entrance to the zones as shownat 70, 71, 72 and 73 in FIGS. 3 and 6.

Curve 38 shows an operating condition that achieves at 5:1 speed ratiowhich increases the separation of carriages in the operational sectioncompared to curve 37 and, therefore, increases the number of carriagesin the stack forming section. This results in a slight increase in thestack length as illustrated by the end-of-stack S-38 shifting to theleft of S-37.

Curve 39 shows an operating condition that achieves a 3:1 speed ratiowhich decreases the separation of carriages in the operational sectioncompared to curve 37 and, therefore, decreases the number of carriagesin the stack forming section. This results in a slight decrease in stacklength as illustrated by the end-of-stack S-39 shifting to the right ofS-37.

In spite of the fact that the hysteresis linear motor used to propel thecarriages on the return sides of the loop is not always synchronous andno feedback is employed, it, nonetheless, is a stable, robust systemunder expected variations in individual carriage driving force orfriction. This is so for the following reasons:

1. The speed of each decelerating carriage is checked to a set value ineach zone as the hysteresis secondary reaches synchronous speed beforeleaving each zone, thereby limiting the time and distance over whichvariations can occur.

2. The carriages are driven into the stack at a constant fixed overspeedthat permits the carriages to catch up, or the stack length to changerapidly, to compensate for variations in uncontrolled variables, therebypreventing the entrance end of the stack from reaching an inoperableposition.

3. The selection of the zone frequency set points in the stack formingsection are chosen to cause the entrance end-of-stack position to beinitially located between the two extremes discussed that would causeoperability problems.

To illustrate the ability of the system to be self-compensating, it isimportant to understand that the total time a carriage spends in thestack forming section is a fixed time for a given operating conditionfor the synchronous operational section. For each carriage leaving theoperational section and entering the stack forming section, a carriagemust leave the stack forming section, enter the stack and thereby returna carriage to the operational section. FIG. 9 shows the effect in thestack forming section of variations of an uncontrolled variable, such ascarriage friction, on the stability of the system.

To set up a base case condition in a model of the system for evaluatingfriction variations, the operational section conditions in FIG. 3 weremaintained and the stack forming section conditions were varied byassuming changes in the value of friction for the carriages. Toexaggerate the effect to a worst case condition, the base case carriagefriction force was assumed to be about 17% of the average carriagedeceleration force. (Ordinarily friction forces are more like 13/4% asthey were for FIG. 3). For this base case the operating speed (relatedto drive frequency) for each deceleration zone EM wave was determined.In FIG. 9, these speed values and other variables were held constantwhile average carriage friction was varied plus and minus 50%. For thebase case with 17% friction, the stack position S-54 was 75.3 lambda.The deceleration curve for the base case is omitted for clarity. Dashedline curve 55 shows the result of decreasing the carriage friction toabout 81/2%. The carriages now do not decelerate quite as rapidly (sincefriction aids deceleration) therefore the carriages reach the stacksooner so the stack grows slightly to compensate by moving left toposition 74 lambda at S-55. With an increased stack, however, thecarriages travel longer at the lower stack speed so the total time inthe stack forming section remains the same and the new stable operatingcondition of curve 55 is established.

Curve 56 shows the opposite condition of assuming an increase in theaverage friction of the carriage to 251/2%. In this case the carriagedecelerates more rapidly and takes longer to reach the stack which hasshortened slightly to compensate by moving right to 76.7 lambda at S-56.Now, however, the carriages travel longer at the higher overspeedvelocity so the total time in the stack forming section remains the sameand the new stable operating condition of curve 56 is established. It isimportant to note that in both curves 55 and 56, the stack entrancepositions S-55 and S-56 did not reach critical limits 0-55, 0-56 or Bfor that operating condition.

In the cases shown here, the acceleration portion is short so the numberof carriages affected by spacing changes is small and the stack entranceposition variations can be measured in a few lambda. In loops having along acceleration portion which may also include a longer constant speedportion than is shown at 44 FIG. 3, many more carriages are involvedbefore reaching the stack forming section, but the hysteresis motorsystem works just as well in this case and can easily accommodate largechanges of one hundred lambda or more in the stack entrance position.

On-Line Changes

During start up from zero speed and during changes from one velocityratio to another as shown in FIG. 8, the EM wave velocities in each zonein the stack forming section must be adjusted periodically duringoperation. This is required to maintain a stable stack entrancecondition as the carriages change energy (speed) and/or spacing enteringthe stack forming section. Updating the stack forming section EM wavespeeds by resetting the drive frequencies to newly calculated valuesabout every 1/2 second will achieve the required stability in the stackforming section. The carriage speed and spacing entering the stackforming section is known from the predetermined operating conditions inthe operational section, which are discussed in the Hommes and KeeganU.S. Pat. No. 4,675,582 referenced previously. System computer 51controls the stack forming section zone drive frequently adjustments andcoordinates them with the operational section operating conditions. Suchfunctions of a computer control system are known to one skilled in thisart so further discussion of control details is not necessary.

System Variations

In the version of the endless loop concept shown in FIGS. 1 and 4, thelinear motor primaries are not continuously acting on either one or theother secondary as a gap exists between where the lower primary ends at74 and the upper primary begins at 32.

In a modified system, along the stack forming section the lower primarycan also have gaps between zones to save the cost of primary and forstructural convenience. Since the carriages are being slowed from anelevated speed, they have sufficient inertia to pass across the gaps ina predictable manner. Some form of auxiliary or operator assist can beprovided in the gaps as desired to insure carriages do not become"stranded" there at shutdown.

The carriage collection section may also contain gaps in zone 19. Forinstance, zone 19 can consist of two straight segments preceding andfollowing the turn-around curve with no primary present in the curvesince curved primaries are difficult and expensive to fabricate. The twoseparate straight segments would be operated together like a single zoneand would be mechanically spaced at the same pitch as the secondaries onthe abutted carriages filling both segments and the curve. The abuttedcarriages would be pushed through the unpowered curve.

The concept of using a carriage having a synchronous and hysteresissecondary has been described where only one secondary was acted on by aprimary at a time. It is also contemplated that whenever the synchronoussecondary is being acted on, a primary could be added to also act on thehysteresis secondary at the same time using the same or a differentspeed EM wave. This would have the advantage of providing additionaldriving force on the carriage and could be used to damp out speedoscillations associated with the synchronous secondary.

It is also contemplated that the synchronous secondary and upper primarycould be eliminated and the lower primary could be extended to propelthe hysteresis secondary completely around the loop. In the carriagecollection section, the hysteresis secondary would be propelledsynchronously to insure a known position versus time for the carriagesat one point in the loop. On the operational side of the loop thehysteresis secondary can be propelled either synchronously, using thesame zones and EM wave segments as were used for the synchronoussecondary, or asynchronously and essentially synchronously in a manneropposite that used on the stack forming side. When the hysteresissecondaries are propelled asynchronously, the percent difference inloading on each carriage at a given position in the system should beminimized to insure predictable performance from carriage to carriage.This is required so the time it takes each carriage to pass through theoperational side is predictable and repeatable for each carriage. Thiswill decrease the possibility of uncontrolled collisions duringdecelerations on the stack forming side.

In a preferred embodiment of this invention linear motors are used forpropulsion throughout, however, the carriages also may advantageously bepropelled by a combination of a hysteresis linear motor and a mechanicalsprocket and/or screw engaging the carriages. For instance, in a filmtenter system, such as that shown in U.S. Pat. No. 3,932,919 toHutzenlamb, for example, a sprocket engages the carriages in a stack andpropels them at a first speed synchronously, in non-slipping engagementwith a drive means whose speed and position can be controlled precisely,and acts as a carriage collection section. The carriages are then passedin abutment (i.e., synchronism is maintained) to a screw with increasingpitch that is synchronously geared to the sprocket. The screwsynchronously spaces the carriages apart and propels them synchronouslyto a second speed, acting as an operational section. At this point, thehysteresis linear motor of the instant invention can be used to engagethe carriages and propel them at the second speed, then return thecarriages back to the stack by decelerating them to a third speed andabut them with the stack at a controlled impact speed, and press thecarriages into the stack, thereby acting as a stack forming section.Such a system would eliminate the costly, complex, and speed limitingsystem (used in this patent) of a chain, decreasing pitch screw, andadjustable pitch screw elements to return the carriages, which elementsmust be changed each time a different increasing pitch screw isselected. To accomplish improved operation, each carriage would receivea hysteresis secondary, and a primary with a plurality of zones andcontrols would be located at appropriate positions along the endlessloop. The hysteresis linear motor would be operated according to theteachings of this invention to return the carriages, to the stack.

Tenter Frame

In the preferred embodiment of this invention, linear motors are used topropel carriages throughout opposed loops of a tenter frame to draw aweb of material, such as plastic film. In operational sections of theloops, individual pairs of carriages are controlled to space them fromadjacent carriage pairs where the film is longitudinally drawn in thefilm processing section of the tenter. A further alternative embodimentexists wherein there are passive carriages introduced between each ofthe carriages actively powered throughout the loops by the linearmotors, the passive carriages being unpowered in the carriage collectionand operational sections of the tenter frame. Both the active andpassive carriages are propelled by linear motors in the stack formingsections.

In greater particularity, and referring specifically to FIGS. 10, 11,12, and 13, an apparatus or tenter frame 901 is shown which is suitablefor drawing a web of sheet material 961, such as plastic film, toimprove its properties. The tenter of FIG. 10 consists of two opposed,mirror-image, symmetrical, clip loops 900 and 902 that move the filmalong forward sides of the apparatus, to transport, draw, and stabilizethe film in the transverse and machine directions simultaneously. Thetransverse direction (TD) drawing is accomplished by diverging the filmclip guide tracks in each loop in a conventional manner. The machinedirection (MD) drawing is accomplished by accelerating pairs of activeclip carriages, such as 906 and 908, with synchronous linear motors toseparate them from adjacent active clip pairs, such as 898 and 899,along the forward sides of the tracks to draw the film between adjacentclips. Unpowered passive clip carriages are placed between the activeclip carriages to decrease film edge scalloping. As the clip carriagesreach the ends of the forward sides of the loops, they are engaged byfriction wheels 993 and 995 on the return sides that open the clips torelease the film edge bead and transport the carriages from the firstprimaries on the forward sides to the second primaries on the returnsides. Both the active and passive carriages are then decelerated withlinear hysteresis motors on return sides and are abutted in stacks,following which the active carriages are engaged again by thesynchronous linear motors before leaving the return sides. The clipcarriages are then recirculated to the entrance of the forward sides forengagement again with the entering film. A control system, supervised bycomputer 1018, precisely controls several hundred active clip carriagessimultaneously and independently on the forward (film) sides of eachloop and brings all clip carriages together without damaging collisionsinto stacks on the return sides.

In the operation of this apparatus a web of film to be drawn is suppliedfrom a supply source 916, such as a supply roll or from a film castingoperation and moved by appropriate means into the tenter frame 901between the pair of elongated endless track loops 900 and 902 positionedopposite each other. The two loops 900, 902 are symmetrical, withelements of loop 900 mirrored in loop 902. Where convenient indescribing elements of the loops, an element will be shown in one loop,and for the other loop it will be referred to by a prime (')designation. The carriages, such as active carriages 906, 908, 927 and928, are propelled along the forward sides 910 and 912 of loops 900 and902 respectively in paired symmetry. That is, the carriage pairs such as906 and 908 are aligned along a line 914 and carriage pairs such as 927and 928 are aligned along a line 925, both lines drawn perpendicular tocenter line 904 centered between the forward sides of the two loops.

Each track loop has a number of portions which fall within the forwardsides 910 and 912 and the return sides 903 and 923 of the tenter frame.First portions of the tracks define a transport section 918 of thetenter frame where the film is conditioned prior to drawing by heatingit to the desired temperature without permitting stretching to occur.Second portions of the tracks define the key drawing section 922 on theforward sides of the tenter frame. These portions of the trackstypically diverge outwardly from a machine center line for drawing theweb in the transverse direction at the same time it is drawn in themachine direction. Third portions of the tracks are connected to thesecond portions on the forward sides of the tenter frame. These portionsare opposite each other, generally equidistant from the centerline ofthe frame, and define between them a stabilizing section 926 of theapparatus. Some small amount of MD and TD drawing or relaxing may occurin the stabilizing section. The dividing line 924 between the drawingand stabilizing sections preferably can be moved up and downstream inthe tenter as film processing requirements vary which will be discussedlater.

These first, second and third portions of the tracks define together thefilm processing section making up the forward sides 910 and 912 of thetenter frame. The tracks are completed on the frame by fourth portionsmaking up the return sides 903 and 923 where the tenter clips aredisengaged from the film. The return sides connect the third portions tothe first portions of the tracks to complete the endless loops.

Referring to FIG. 10, a plurality of active carriages such as 927, 928,896, and 897 are positioned for movement on the elongated track loops900 and 902. A plurality of passive carriages such as 931 and 933, 935and 937 are positioned between active carriages 927 and 897, and 928 and896 respectively as shown. Both active and passive carriages have tenterclips 959 attached to them, as best seen on the active carriages in FIG.12, which are adapted to grip the edges of the film as it enters thetenter frame 901 and to release the film after it has been moved by thecarriages through the forward sides. On the forward sides of the tenter,the passive carriages are first propelled by entrapment between theactive carriages and are then propelled by the attached film as theactive carriages separate and stretch the film. After releasing thefilm, both active and passive carriages are propelled along the returnsides of the tenter frame into position to repeat the drawing operation.FIG. 14 shows the active and passive carriages, such as 927, 931, 933,and 897, as they would appear in the transport section on the forwardsides of the tenter where they are abutted and are engaging the edgebead of the unstretched film. In FIG. 15, the same carriages are shownas they would appear in the drawing or stabilizing sections of thetenter where the active carriages have separated, thereby stretching thefilm, and the passive carriages, due to their gripping engagement withthe film, are propelled by the moving film itself. The intermittent linein FIG. 15 depicts the excessive scalloping of the film edge that wouldoccur if the passive clips were not present.

The active clip carriages have synchronous secondaries attached to themthat are electromagnetically engaged on the forward sides and the endsof the return sides by the mechanically separable first primaries 975,977, 979, 981, 983, 985, 987, 989, and 991 in loop 902. Loop 900 hascorresponding first primaries in mirror image positions to loop 902. Thelinear snychronous motor control system previously described is appliedto the first primaries to control the propulsion of the activecarriages, such as 906 and 908 along the forward sides 910 and 912 ofthe track paths. Computer 1018 supervises control of the system. The twoforward sides are connected and coordinated in a manner to provide totalcontrol over each opposed pair of clips as they move through the drawingsection and other film processing sections.

The active and passive clip carriages both have hysteresis secondariesattached that are electromagnetically engaged on the return sides ofboth loops by the machanically separable second primaries 939, 941, 943,945, and 947 in loop 902. Loop 900 has corresponding first primaries inmirror image positions to loop 902. Paired groups of coils in the secondprimaries on the opposed loops receive the same operating instructionsfrom computer 1018 for control purposes. The linear hysteresis motorcontrol system previously described is used to control the propulsionand stacking of the active and passive carriages and maintains pressureabutment of the carriages in the stacks as the synchronous secondarieson the active carriages are engaged again by the first primaries, suchas 975 in loop 902, before leaving the return sides.

In a typical application, the web of material or film 961 is formedupstream at 916 and is fed to the tenter frame entrance at 920. Thetenter clips on the opposed active pairs of carriages grasp sequentiallysuccessive areas along opposite edges of the film at 920 and propel itat a first constant speed through the transport section 918 where thefilm is heated without drawing. The passive clip carriages interposedwith the active ones in the transport section also grasp the film edgesand are carried along by abutment with the active carriages. The tracksthen diverge at 921 thereby drawing the film transversely while at aboutthe same time the opposed active pairs of carriages are individuallyaccelerated causing them to separate from adjacent pairs andsimultaneously draw the film longitudinally in the drawing section 922.Heating of the film in the oven enclosure 895 is continued in thedrawing section to control the film temperature during drawing. Thetracks are then made generally parallel and the individual opposed pairsof carriages reach a second speed at the end of the drawing section at924 and the film is stabilized in section 926. The speed of thecarriages and draw ratio of the film may change slightly in thestabilizing section. Temperature control of the film continues in thestabilizing section and may consist of continued heating or cooling. Thefilm is then released from the tenter clips at the tenter frame exit at929 and continues to a conventional winder. The individual active andpassive carriages in each loop are then returned along return sides 903and 923 of the two endless carriage loops to the entrance 920 of thetenter frame.

If it is desired to slacken or relax the film in the machine directionin the drawing or stabilizing sections, the speed of the activecarriages as controlled by the linear motors may be gradually orprogressively decreased slightly according to any desired programthereby providing direct control of the shrinking and flatness of thefilm. TD relaxation is also possible in the stabilizing section byslightly converging the tenter frame tracks as shown at 938 to move thetenter clips closer together laterally.

The symmetry of motion between carriages along the forward sides of loop900 and loop 902 is assured by:

providing a linear motor primary adjacent each track, with each primaryincluding a plurality of groups of coils with the groups of coils in oneprimary sized to match opposed groups of coils in the other primary andwith each of the opposed groups of coils being electrically joined anddefining a single control zone;

providing a synchronous secondary attached to each of the activecarriages, the attached secondary guided adjacent one of the primaries;

providing a continuous supply of closely spaced or abutted activecarriages to each loop initially in synchronism with matchingelectromagnetic waves developed in the groups of coils in a control zoneat the entrance of the tenter frame; and

providing predetermined coordinated control instructions simultaneouslyfor all coil groups in each control zone of the opposed primaries tothereby develop predetermined coordinated EM waves in all control zonesso the opposed pairs of active carriages in the two loops are propelledin symmetry through each control zone and from one control zone to thenext through the tenter frame.

The synchronous secondaries lock onto the electromagnetic wavesdeveloped by the primaries and as long as the active carriages arecontinuously fed in alignment to the forward sides of the loops, and theopposed groups of coils in each control zone simultaneously receivealternating current developed from the same predetermined controlinstructions, which are simultaneously coordinated with adjacent controlzones' instructions, the active carriages will remain in symmetry asthey are propelled along the forward sides of both loops.

The operation of propelling the tenter carriages around a single loop issimilar to the system shown in FIGS. 4, 5 and 6. The control of carriagepropulsion along a forward side of each loop by predetermining andcoordinating the control zone waveforms is described in our previouslymentioned U.S. Pat. No. 4,675,582.

FIG. 4, represents one of the two loops of the tenter frame forpropelling the carriages. The carriage collection section 41 andoperational section 42 of FIG. 4 are operated as previously described tocontrol the propulsion of the active carriages through the carriagecollection sections 949/949' and through operational sections 951/951'on the forward sides 910 and 912 of FIG. 10. The stack forming section43 is operated as previously described to control the propulsion of bothactive and passive carriages throughout the stack forming sections953/953' on the return sides 903 and 923 of FIG. 10. The carriagecollection sections 949/949' are operated to propel the active carriagesthrough the remainder of the return sides.

The control zones for the tenter frame loops comprise opposed groups ofcoils that are electrically joined. The groups of coils are shown asblocks in the first and second primaries of loop 902. For clarity inFIG. 10, only loop 902 has the primaries and coil groups shown, but theyare also present in mirror image positions in loop 900, and fordiscussion purposes are designated by the number in loop 902 with aprime (') suffix. By opposed groups of coils then is meant, forinstance:

the group of coils 930 in first primaries 977 and 979 of loop 902 whichare electrically joined with the group of coils 930' of thecorresponding first primaries 977' and 979' of opposed loop 900 whichwould make up a control zone A;

the group of coils 932 in first primary 983 of loop 902 which areelectrically joined with the corresponding group of coils 932' in firstprimary 983' of opposed loop 900 which would make up a control zone B;

the group of coils 934 in first primary 985 of loop 902 which areelectrically joined with the corresponding group of coils 934' in firstprimary 985' of opposed loop 900 which would make up a control zone C.

It is important to note that the groups of coils 930, 930' are from twomechanically separable first primaries. The significance of this will bediscussed later.

The groups of coils of the first primaries just discussed are adjacentthe synchronous secondaries on the active carriages along the forwardsides of the loops. They correspond to zones such as 19, 20, 21 and 22in FIG. 4. Wherever the carriages are to be spaced apart, as in theoperational sections 951/951', the control zones are sized so that for arange of desired operating conditions, there will never be more than onepair of active carriages at a time in a control zone as the activecarriage pairs are propelled in symmetry along the first primary. Thiscondition does not apply, however, in the first constant speed controlzones, such as 981, 981' (FIG. 10), where the carriages are all closelyspaced or abutted at an integral multiple of lambda and are traveling atthe same speed; there can be many carriage pairs in these control zones.

Tenter Carriage, Track, and Linear Motor

FIG. 11 shows a typical cross-section taken along lines 11--11 in FIG.10 through the forward and return sides of the loops 900 and 902. Shownin elevation are active carriages, such as 927/928 on the forward sides910 and 912, and 886/887 on return sides 903 and 923 respectively. Ovenenclosure 895 encloses much of the forward sides to control filmheating, while track enclosures 888 and 889 enclose much of the returnsides to facilitate control of clip temperatures to prevent sticking to,or quenching of, the film when initially grasped by the clips. The trackenclosures also function as safety guards for the moving carriages.

FIG. 12 shows an enlargement of view 12 in FIG. 11 through the forwardside 910 of loop 900. Active carriage 927 is shown in side elevation.Enlarged plan views of the active and passive carriages are shown inFIGS. 14 and 15 and front elevation views are seen in FIG. 16. A guidetrack such as 942 is provided that runs completely around loop 900 alongboth forward and return sides and connects the forward and return sides.The track is supported by attachment to frame 962. The track is aflexible structure to permit smooth curving at inflection points and isshown for example in U.S. Pat. No. 3,456,608, incorporated herein byreference, and U.K. 1,504,450. The carriage is supported on guide track942 by the eight rollers 944, 946, 948, 950, 952, 954, 956 and 958 thatare rotatably mounted on carriage body 960. The rollers are alternatelyaligned and offset in the MD or longitudinal direction (into FIG. 15) toprovide a stable support for the carriage. That is, also referring toFIG. 16, horizontal track surface rollers 944 and 958 are longitudinallyaligned, while horizontal rollers 946 and 956 are also longitudinallyaligned but are longitudinally spaced, or offset, from rollers 944 and958. Likewise, vertical track surface rollers 948 and 952 arelongitudinally aligned, while vertical rollers 950 and 954 are alsolongitudinally aligned but are longitudinally offset from rollers 948and 952. Other numbers of rollers or sliding elements may be utilized aslong as the carriage is stably supported for free sliding or rollingmovement along the guide track. Alternate track arrangements are alsopossible. The rollers keep the carriage closely positioned on the trackand carry loads produced by the weight of the carriage, the tension ofthe film 961 grasped by the film clip, the thrust of the motor, and theunbalanced magnetic forces between the primary and secondary.

As best seen in FIGS. 14 and 15, two passive carriages, such as 931 and933, fit between every two active carriages, such as 927 and 897. Thepassive carriages are similar to the active carriages but withoutsynchronous secondaries attached. As such, the passive carriages fitvertically between the overhanging synchronous secondaries of the activecarriages when the active carriages abut with typical stops 997 and 999of carriages 927 and 897. The horizontal and vertical rollers on eachcarriage are staggered as previously described so "nesting" of therollers occurs between the adjacent carriages. If smaller rollers wereused or a greater spacing between carriages were employed, such"nesting" would not be required. To achieve large draw ratios, however,close initial spacing of the film clips is desired to minimizescalloping.

Referring to FIGS. 14 and 15, each carriage has an elastomeric bumper toabsorb the controlled impact that occurs as the carriages abut in thestack forming sections of the loops. Low level impacts may alsooccasionally occur within a return side control zone as a carriagefriction load may vary slightly or the air gap between the hysteresissecondary and return side primary may slightly vary from one carriage toanother. Referring to the hidden lines in FIGS. 14 and 15, activecarriage 927 has a bumper 1001 and impact surface 1003. Passive carriage931 has a bumper 1005 and impact surface 1007. In operation, adjacentbumpers and impact surfaces, such as bumper 1005 and surface 1003 cometogether as shown to cushion impact between adjacent carriages. Onlywhen all intervening bumpers between active carriages 927 and 897 aredepressed some finite amount do stops 997 and 999 on carriages 927 and897 come into contact as in FIG. 14.

Film Clip

Both the active and passive carriages have film clips 959 attached astypically shown in FIG. 12. Carriage body 960 has attached a film cliplever 964 pivotably connected at 966. A gripping surface 968 ispivotably movable to clamp the film against anvil surface 970 whichstops the pivoting movement. The film 961 is gripped by forcing thelever 964 in the direction of arrow 972 and is released by forcing thelever in the direction of arrow 974. The upper end of lever 964 formscam following surfaces 963 and 965. In FIG. 10, cam surfaces 967 and 969at the entrance of the tenter frame act on surface 965 on the carriagesto move the lever to grip the film, and the peripheral surfaces of thefriction wheels 993 and 995 at the tenter frame exit act on surface 963on the carriages to move the lever to release the film. Suitable tensiondevices such as springs may be connected between lever 964 and carriagebody 960 to maintain the clip in the open and closed position such thatthe clip is forced into an opposite position only under the action ofthe cam surfaces. This arrangement is preferred so that cams 967, 969,and the friction wheels only need be placed at the entrance and exit ofthe tenter frame to open and close the clips. The longitudinal dimension(into FIG. 12) of surface 968 and anvil 970 is narrow to permit freelongitudinal movement of the film as it is stretched between clips. Ithas been found that the film also stretches longitudinally where it isgripped by the clip. Clips for simultaneous biaxial stretching of filmare disclosed in the previously mentioned tenter frame patents and U.S.Pat. No. 3,391,421 and need no further explanation.

Secondaries

In FIG. 12, on the top and bottom of active carriage body 960 areattached synchronous secondaries 976 and 978. They resemble thesynchronous secondaries described in FIG. 2 and in U.S. Pat. No.4,675,582 (previously incorporated by reference). In FIG. 12, they eachconsist of magnets located at 980 and 982 and back iron at 984 and 986similar to the secondary 3 shown in FIG. 2. As better shown in FIGS. 14and 15, the magnets at 980 would consist of one magnet oriented with itsnorth pole facing outward and a second adjacent magnet, spaced onelambda away, with its south pole facing outward. As is shown in FIG. 14,referring to secondaries 927 and 897, it is preferred that secondarieson adjacent active carriages have the disposition of their polesreversed. Also when adjacent active carriages are at their closestspacing, which in the tenter frame invention is with active carriagebodies abutted, the magnetic edges of the secondaries would have a onelambda space between them as is explained in the '582 patent. Referringto a single active carriage such as 897, the magnet near the forwardside of the carriage on the top secondary 976 and the magnet near theforward side of the carriage on the bottom secondary 978 would both havetheir outward facing poles the same. For example, both top and bottomsecondaries would have north poles near the forward side of thecarriage. As is shown in FIGS. 14 and 15, the magnets are preferablyangled with respect to the track centerline 1009. This helps to removeforce fluctuations on the moving carriages that are caused by attractionof the magnets to the teeth of the laminated primary which are orientedperpendicular to the track centerline as seen in FIG. 21. The magnets onthe top of the active carriage are preferably oriented opposite theangle of the magnets on the bottom of the carriage. The magnets are alsopreferably surrounded on the top and sides by a non-magnetic,electrically conductive cage 1011 (FIG. 14), such as copper or aluminum,to contain the magnets and provide dynamic electromagnetic damping tothe moving cartridges (a thin top covering is omitted in the figures forclarity). The one lambda pole-free space between active carriages isphysically established by the abutting stops 997 and 999 on adjacentactive carriages 927 and 897.

Referring to FIG. 12, at position 940 on the angled surface of theactive carriage beneath the film clip is a hysteresis secondary whichresembles the hysteresis secondary discussed in reference to FIG. 2. Itconsists of hysteresis material 936 mounted on the back iron comprisingthe carriage body 960. At this same relative position on the passivecarriages, there is also a hysteresis secondary. The hysteresis materialis preferably unmagnetized Alnico V and, as shown in FIG. 12, it is thesame thickness and height on both active and passive carriages. FIG. 13is an enlargement of view 13 in FIG. 11 through the return side 903 ofloop 900. Referring to FIG. 13, in the position 940 on both the activeand passive carriages, the hysteresis secondary is adjacent the secondprimary (943') for engagement by the second primary EM wave. Since theactive and passive carriages frequently have a different mass andtherefore a different kinetic energy at the same speed, the size of thehysteresis secondaries on the respective carriages is different as canbe seen in FIG. 16 which shows an elevation view taken along line 16--16of FIG. 15. The hysteresis secondaries are shown in crossed lines whereit can be seen that the active carriage has a wider area 1021 than thepassive carriage area 1023. When both secondaries are engaged by thesame second primary EM wave, a larger force is developed on the activecarriage than the passive one. This larger force is proportional to thegreater mass of the active carriage and causes both carriages to beslowed (negative acceleration) at the same rate since a=F/m.

Aligning the Active Carriage Secondaries

The active clip carriages must enter the tenter frame at a known spacingand in synchronism with the EM wave in the first control zone, A(reference coil groups 930/930'). Prior to machine start-up thecarriages are pressed up against one another with the carriage stopsabutted and the lead active carriage held stationary at, say 921. Thisestablishes the spacing at a known unvarying value at which the magneticpole pitch of the carriage secondaries matches the EM wave pole pitchdetermined by the coils in the primary, such as first primaries 977/979and 977'/979' comprising control zone A. A variety of means can be usedto press the carriages together, such as linear motor means, conveyorbelt means, gravity means, etc. In the preferred embodiment of theinvention, linear hysteresis motor means on return sides 903/923 areemployed. Since the preferred apparatus has two different polarityactive carriages, as discussed earlier, the proper polarity activecarriages must be located in both forward sides 910/912 at 921. Forinstance, if the starting EM wave requires a north magnet near theforward side of the carriage at 921 in the cartridge collection sensorof forward side 910, then a north magnet is also required near theforward side of the carriage at 921 in the carriage collection sectionof forward side 912. This special alignment is necessary whenever themagnet polarity on adjacent carriage secondaries is reversed as ispreferred. Obviously, when alternate polarity carriages like these areused, there must always be an even number of carriages in each loop sothis alternating polarity reversal is maintained as the carriagescontinuously circulate in the loops.

This preferred pre-start-up orientation of carriages can be achieved bymanual positioning of the carriages or by proper operation of the linearmotor control system by computer 1018. The sequence of steps utilizingthe control system is as follows:

1. Beginning segments of the first primaries, such as in the transportsection, are turned off; then computer 1018 causes second segments ofthe first primaries, such as in the drawing section, to be energizedwith the proper polarity DC current at a low value causing the coils ofthe three phases to develop stationary, alternated poles of an EM wavealong the second segments.

2. Computer 1018 causes the remaining first primaries, the frictionwheels and the second primaries to slowly propel all carriages out ofthe draw and stabilize sections and stack them on the return sides. Thehysteresis linear motors on the return sides push the carriages throughthe transport section and into the draw section until the combined forceof the stationary poles acting on several abutted carriages in the drawsection is sufficient to overcome the pressing force of the hysteresismotors. The carriages along the second segments are held stationary, orfixedly engaged, by the EM wave.

3. Computer 1018 increases the current to the first primaries in thedraw section to the normal operating level. There should be a briefdelay while the carriages move back on the EM wave.

4. Computer 1018 causes the first primaries in the transport sections tobe energized with the proper polarity DC current causing the coils todevelop a stationary EM wave to engage the abutted carriages there tohold them stationary in phase, or "locked-up" on the EM wave.

5. Computer 1018 causes the primaries in the draw section to turn off;

6. Computer 1018 checks sensors at the end of the transport section atabout position 921 to find out the type of active carriage there (northpole forward or south pole forward) in each forward side. The sensor candetect physical features provided on the carriages. If both carriagesare the proper type go to step 9.

7. If the carriage at 921 on forward side 912 is incorrect, advance thephase polarity only on forward side 912 to step the carriages forwarduntil the proper carriage is detected at 921 in forward side 912;

8. If the carriage at 921 on forward side 910 is incorrect, advance thephase polarity only on forward side 910 to step the carriages forwarduntil the proper carriage is detected at 921 in forward side 910;

9. Computer 1018 causes the first primaries in the draw and stabilizesections, the friction wheels, and the second primaries on the returnsides to slowly propel the unwanted carriages out of the forward sideand into the stack on the return side;

10. The system is now initialized and the carriages can start running atthe start-up speed at the desired start-up simultaneous stretch profile(it should be noted in some instances, the start-up stretch profile andthe final stretch profile may differ as explained later);

11. After the carriages are running, the film can be threaded up at thetenter entrance in a conventional manner.

After this pre-start-up orientation of carriages is established, and theremainder of the forward sides of the loops are empty of carriages, thetenter frame can be started up and the carriages will be propelled oneafter the other along the forward sides in synchronism with the EM wavesand returned along the return sides as shown in FIG. 10. This is similarto the operation of the single endless loop shown in FIG. 4 and thispre-start-up system is also useful for such a single loop. If theforward sides of the tenter frame are stopped in a controlled fashion,the relative positions of the carriages can be maintained and restartingdoes not require realigning of the carriages.

Primaries

Referring to FIGS. 10, 11 and 12, the first elongated primaries of thetenter frame loops are present in the carriage collection andoperational sections of each loop. These first primaries, such as985/985' comprised of upper primaries 985U/985'U and lower primaries985L/985'L, interact electromagnetically with the upper and lowersynchronous secondaries respectively on each active carriage. In FIG. 4,these first primaries correspond to the upper linear motor primaries 5.Second elongated primaries of the tenter frame loops are present in thestack forming sections of each loop. In view 13 of FIG. 11, the secondprimaries 943/943' are diagonally positioned to interactelectromagnetically with the diagonally positioned hysteresissecondaries on the active and passive carriages. In FIG. 4, these secondprimaries correspond to the lower linear motor primaries 6. Thearrangement of these tenter frame first and second primaries intocontrol zones and the control of such control zones is similar to thearrangement and control of, respectively, the zones of the upper andlower primaries 5 and 6 discussed with reference to FIG. 4.

A typical primary structure is shown in FIGS. 2, 12, 13, 21 and 22, andin FIG. 2 of U.S. Pat. No. 4,675,582, and consist generally of coilsplaced in slots between laminated metal teeth. The predeterminedalternating current to the primaries is supplied via conductors such asat 992 and 994. The use of both upper and lower primaries along selectedportions of the forward side provides maximum thrust to an activecarriage by simultaneous propelling of both of its synchronoussecondaries 976 and 978. In some sections of the tenter frame wheremaximum thrust is not required, it may be convenient to omit one of theupper or lower primaries, for instance, the upper of the first primaries977, 979, 981, 989, and 991. However, the upper and lower primaries alsoprovide a balanced magnetic attraction force on the carriage so when oneis omitted, the vertically disposed rollers on the carriage become moreheavily loaded and the bearings must be sized accordingly. The primariesare positioned to be closely spaced from the carriage secondariesseparated only by clearance gaps 996 and 998 in FIG. 12 and gap 1013 inFIG. 13.

In FIG. 12, forward side frame 962 also includes support plates for theupper and lower primaries 985'U and 985'L when both are present. Theseplates directly contact the back of the primary, and each have channelssuch as 1017 and 1019 for circulating cooling liquid to keep theoperating temperature of the primaries down. Each primary, such as 985,has its core and end coils potted to provide mechanical protection tothe coils and to conduct heat efficiently from the coils to the cooledsupport plates. Potting compounds such as filled epoxies, silicones, orceramics are suitable as long as they have a temperature resistance andthermal conductivity compatible with the thermal loads imposed on theprimaries. A preferred potting procedure is to surround all but one sideof the motor with a steel frame, fill the motor with alumina oxideceramic grit, and then apply a one part or two part epoxy so the percentfill by volume of the filler is about 80 percent. Such a technique isknown for potting transformers. A two part epoxy having a bisphenol-Aresin and nadic methyl anhydride (NMA) hardener has been successfullyused. The frame preferably remains as an integral part of the motor.Heat in the primary comes primarily from I² R electrical losses and fromheat absorbed from the film heating ovens. Strategically placed shieldsmay provide some additional protection from radiant oven heat.

Wedges

Along the forward side of the tenter, there are several positions wherea first primary begins or ends such as at 920 for the lower primaries,at 921 for the upper primaries, at 892 for the upper primaries, and 929for the lower primaries. At these positions, a synchronous secondarygoes from having air adjacent it to having the steel laminations of theprimary core adjacent it. At these positions, it has been foundbeneficial to provide metal wedges such as those shown in representativeloop 902 at 1060, 1062, 1064, and 1066. These wedges minimize theundersirable effects of a synchronous secondary magnet approaching theleading end and leaving the trailing end of a first primary. Sucheffects and details of the wedge construction and function are describedin co-pending application, Ser. No. 209,909 filed June 22, 1982 now U.S.Pat. No. 4,922,142 filed concurrently herewith, which application isherein incorporated by reference. Along the return side of the tenter,there are several positions where there are gaps where a carriage has nocontrollable propulsion means acting on it. Referring to loop 902 inFIG. 10, such gaps occur between the exit of the friction wheel 995 andsecond primary 939 and at inflection points such as between secondprimaries 939 and 941, 941 and 943, 943 and 945, etc. At these gaps, ithas been found beneficial to provide metal double-wedges such as thoseshown in representative loop 902 at 1068, 1070, 1072, 1074, and 1076.These double-wedges, such as 1072' in FIG. 13, are adjacent the lowersynchronous secondary 978, and they act on the synchronous secondary toprovide propulsion across the gaps at low speeds when carriage inertiamay be insufficient to carry the carriage across the gaps. To minimizethe induced magnetic resistance the double-wedge presents to the motionof the carriage at high speeds, the double-wedges are preferablyconstructed of thin sheets laminated together similar to the primarycore. The operation and details of the double-wedge construction arealso described in the above mentioned co-pending application.

Inflection Points

Referring to FIG. 10, there may be a number of inflection points in theloops where the track and the first primaries make an adjustable angularchange from a straight path. These present a problem in maintainingprecise control of carriage propulsion since the first primaries areinterrupted at an inflection point. These occur, for example, at 921between the transport and drawing sections and at 924 between thedrawing and stabilizing sections on the forward sides of the loops.There may also be inflection points at other forward side locationswhere the tracks are moved to accommodate angle changes when the tenterframe is adjusted to fine tune TD draw ratios. Inflection points in thetracks also occur at corresponding positions on the return sides, butsince the degree of control of carriage motion is less precise on thereturn sides there is no problem presented there. FIGS. 21 and 22 showenlarged views of an inflection point such as at 921 in forward side910. At the inflection point it has been found most convenient to omitthe primary coils so flexing can freely occur and the wires in the coilsare not subjected to repeated bending that eventually causes fatigue andbreakage. Omission of the coils also makes it possible to removesections of primary for maintenance and repair without disturbing theremainder of the primary. The loss of electromagnetic force at aninflection point with omitted coils can be minimized by taking thefollowing steps:

eliminating one phase-set of coils at the inflection point;

powering the coils on either side of the inflection point with the samepower waveform;

providing overlapping back-iron at the inflection point.

A significant aspect of the second step is that a control zone boundarywill never occur at an inflection point since both sides of theinflection point are powered by the same waveform, i.e. one from asingle control zone.

FIG. 22 shows a section view of the inflection point of FIG. 21. Eachprimary consist of thin steel laminations, 1015, glued and boltedtogether to form alternating slots, such as 1020, and teeth, such as1022, and black iron, 1024. Coils of wire, such as 1026, are placed inthe slots as shown. In this example where three phase AC power is usedto energize the coils, adjacent coils are A, C, and B phases making upone phase-set of coils. Primary 981' can pivot clockwise orcounterclockwise with respect to primary 983' about pivot center 1028.The ends of each primary at the inflection point are equally chamferedat 1030, 1032, 1034, and 1036 to provide clearance during pivoting.

Primary 981' has the teeth and part of the back-iron cut off at 1038.Primary 983' has some of its back-iron removed at 1040. The back iron ofprimary 983' at 1040 can therefore overlap the back-iron of primary 981'at 1038 as shown. The back iron ends at 1042 and 1044 are chamfered, asthe ends of the primary were, to provide clearance during pivoting.There is a small clearance gap 1058 between the overlapped sections ofback-iron so they can pivot freely without binding when the primariesare mounted on support plates 1046 and 1048. The support plates alsohave chamfers for clearance during pivoting.

At the inflection point, three adjacent coils have been removed (shownin phantom in FIG. 22) from the normal three phase progression of coilshad there been no inflection point. That is, an A, C and B phase coilare missing at the inflection point, i.e. one phase-set of coils. Itshould be noted that the break or joint between primaries at theinflection point occurs at a slot and not at a tooth. The toothpreferably remains intact since it is an important element in the fluxpath to the secondary. To minimize the distortion in the EM waveform,the current in the same coils on either side of the inflection pointmust have the same frequency, phase, and amplitude. This will producethe proper "consequent pole" at these teeth. To insure this condition,the coils on both sides of the inflection point are preferably poweredby the same source of three phase AC; this is, they are part of the samecontrol zone. For instance, all the coils in the segment of primary 981'shown to the left of the pivot in FIG. 22, and the A coil at 1045, the Ccoil at 1047, and the B coil at 1049 to the right of the pivot would bein the same control zone. This eliminates possibilities of phase shiftsthat might occur if each section of primary on either side of theinflection point were driven by their own separate three phase powersources. This clarifies the earlier discussion of why the group of coils930 making up representative control zone A are from two mechanicallyseparable first primaries, 977 and 979 that meet at an inflection point.

Friction Wheel

FIG. 17 shows a typical section through exit friction wheel 995 such asat 17--17 in FIG. 10. The rotating wheel has an upper level diameter1050 that serves to contact the cam following surface 963 on approachingtenter clip 959 to force it open (as shown) as the active and passivecarriages pass under the wheel. This releases the film from the clip. Alower level diameter 1052 consists of an elastomeric ring 1954 thatforcibly engages a back surface 1056 on both active and passivecarriages. The carriage track 942 has the same center of radius as thewheel and holds the carriage against ring 1054 as the friction wheel andcarriage travel together in non-slipping engagement for about 180degrees of rotation of the wheel. After about 180 degrees of rotation,the guide track 942 straightens out and guides the carriages away fromthe friction wheel. Both friction wheels 993 and 995 are driven, viamechanical gearing, by a single rotary motor 103 whose speed isregulated by computer 1018 to operate in a predetermined manner torotate the friction wheels so that the surface speed of lower diameter1052 closely matches the predetermined linear speed of the carriages asthey exit the tenter forward side. Since the engagement between diameter1052 and the carriages is frictional and not fixed, as by teeth on agear or sprocket, an exact speed match and position match are notrequired and any spacing of carriages can be accommodated. This is animprovement over other known simultaneous biaxial film stretchers,powered by linear motors or other means, where the carriage spacing and,therefore, the longitudinal draw ratio, is fixed by a particular exitsprocket.

Control Zones

FIG. 18 shows an exemplary diagram of the overall control system of theinvention which is based on the the control system of U.S. Pat. No.4,675,582. System computer 100, corresponding to system computer 1018 inFIG. 10, communicates with all of the first primary drivers and secondprimary drives and the friction wheel drive via communication bus 102.Sensor 101 feeds information to the computer about the speed of the filmentering the tenter so the tenter can be coordinated with the precedingpart of the film line.

Along the forward sides of the tenter frame in the carriage collectionand operational sections, each first primary includes a plurality ofgroups of coils with the groups of coils in one primary in one loopsized to match the groups of coils in the other primary in the opposedloop and with each of the opposed groups of coils being electricallyjoined and defining a single control zone. Such control zones areindependently controlled and the groups of coils therein all receivematching or the same driver instructions simultaneously. In the previousdiscussion of the linear synchronous motor system of FIG. 4 for theupper primary, a zone consisted of only a single group of coil windingswhich required a zone driver and a zone controller of independentcontrol of that single group of coil windings. A control zone asdiscussed in describing the tenter frame consists of opposed groups ofcoil windings.

A control zone for the first primaries of both loops may consist of fourgroups of primary coils in the areas of the tenter where high drawingforces are required. Referring to control zone C comprising groups ofcoils 934/934' (FIG. 10), it consists of a first group of upper coils934'U and a second group of lower coils 934'L in operational side 910,and a third group of upper coils 934U and a fourth group of lower coils934L in operational side 912. Referring to FIG. 11, these groups ofcoils are part of upper first primaries 985U/985'U and lower firstprimaries 985L/985'L. The groups of coils in control zone C areelectrically joined to control zone driver means 1000. This driver meansmay consist of a single zone driver such as 136 shown schematically inFIG. 18 and in detail in FIG. 20 or two zone drivers, one for the upperand lower coil groups in each forward side, or four zone drivers, one ofeach group of coils in the control zone. The choice of how many zonedrivers to use depends on the power match between the power requirementsof each group of coils versus the power rating of a zone driver. Thegroups of coils may be electrically connected to the control zone driveror drivers in series, parallel, or series/parallel which also depends onthe above power match. What is important is that a control zonecontroller means such as 1002 for control zone C be common for all thecontrol zone drivers for that zone. This control zone controller meansmay be a single control zone controller as at 128 in FIG. 18, or, forcontroller capacity reasons, it may be two or more control zonecontrollers that have identical control instructions stored in each.When, in a preferred embodiment, control zone driver means 1000 consistsof a single control zone driver, and zone controller means 1002 consistsof a single control zone controller, and the coils such as 138 in FIG.18 represents all four groups of coils in the tenter frame control zone,then FIG. 18 depicts a representative portion of the control system forthe linear motor tenter frame.

It is also contemplated that several control zones can be utilized topropel the carriages the same where the instructions for those controlzones are all the same. For instance, in the transport section where thecarriages are traveling abutted at the same speed, there can be severalcontrol zones, such as control zone A, powered by separate control zonedrivers and zone controllers. These zone controllers, however, would allprovide the same instructions simultaneously to the control zone driverswhen those control zones are all operated to propel the carriages thesame. For special situations, such as carriage initialization, theindividual control zones can also be operated independently.

Just as representative control zone C is connected to a zone drivermeans and zone controller means, so are representative control zonessuch as A and B connected respectively to zone drivers 1004/1006, and1008; and zone controllers 1010/1012, and 1014. Similarly to FIG. 18,all the zone controllers are connected to a central controller 1016which corresponds to central controller 108 and central programmabletime base 106 in FIG. 18. The central controller and zone controllersare also in communication with system computer 1018, corresponding tosystem computer 100 in FIG. 18, which for clarity in FIG. 10 is shownonly in communication with central controller 1016.

The control zone for the second primaries are similar, but not identicalto the control zones for the first primaries. A control zone D for thesecond primaries, such as primaries 943/943', preferably consists of twogroups of coils, such as 890 in the return side of loop 902 and 890' inthe return side of loop 900. These two groups of coils would receiveidentical control instructions. The groups of coils may be connected toa single drive and drive control or each may have a separate drive anddrive control with the two drives and drive controls receiving identicalcontrol instructions. As shown in FIG. 10, coils 890 would be powered bydrive 1078 and drive control 1086, and coil group 890' would be poweredby drive 1080 and drive control 1088; both drives receiving identicalcontrol instructions from computer 1018. Similarly, coil groups 894 and894' would be connected to drives and drive controls 1082, 1090 and1084, 1092 respectively and would make up control zone E.

The above arrangement is reflected in the schematic of FIG. 18 whichshows how two typical second primary zone coils 143 and 145 areconnected to drives and drive controls 141, 139 and 147, 149respectively for communication with system computer 100, correspondingto computer 1018 of FIG. 10. Since the second primary control zones areoperated at a constant frequency and there can be many carriages in azone at a time, the control requirements are much more simple that thefirst primary control requirements. Therefore, there is no need for acentral controller to precisely coordinate the instructions to themultiple control zones of the second primaries. The control zones of thesecond primaries do, however, still cause the carriages to move inapproximate symmetry from one loop to the next, but exact symmetry ofmotion of pairs of carriages will normally not take place nor it isrequired.

On-line changes to the instructions for the second primaries toaccommodate changes in the line speed, or changes in the MD draw ratio,of the tenter can be accommodated by updating the instructions at fixedintervals by computer 1018. This updating would occur during theinstruction changes taking place for the first primaries in a transitionoperation. This updating was previously discussed when referring to thesingle loop linear motor control system. A suitable update interval inthe tenter, where the film will only tolerate gradual changes, if fromabout 1/2 second to about 10 seconds, depending on operating speeds.Between modest operating condition changes for the first and secondprimaries, the system should be allowed to operate steady-state for aperiod to stabilize the system before making further changes. Whenupdating the instructions for the second primaries, all instructions canbe changed simultaneously without significant upset to the carriagesalready in the return side, or just entering it, due to the inherentstability of the linear hysteresis motor system.

Zone Drivers

It is important that sufficient current be present in each control zoneto propel the carriages around the loops. A zone driver utilizingvoltage control suitable for this purpose is described in our previouslymentioned. U.S. Pat. No. 4,675,582.

While this driver is very effective in propelling the carriages, it hasbeen found that a driver utilizing current control can also be used toassure that there is always adequate current in the zones. The driver ofFIG. 8 in the '582 patent can be appropriately modified for thispurpose. Such improved driver, which is shown in schematic form in FIG.20, provides rapid response constant current control for this system. Asimilar type rotary motor driver, with the addition of a flux responsivecircuit not needed by the invention, is also described in U.S. Pat. No.4,259,620, which is hereby incorporated herein by reference.

The problem the modified driver solves is the following. When a pair ofsynchronous secondaries propelled in the dual loops of the tenter enteran empty (no secondaries present over the coils) control zone, twothings happen to vary the current. One, it is believed a back-emf isinduced in the coils of that control zone and two, the inductance of thezone is increased by the presence of the secondary over the coils. Theseeffects cause a sudden momentary drop in the current flowing in thecoils of that control zone and consequently reduces the electromagneticforce engaging the secondaries. This problem is more pronounced at highsecondary (carriage) speeds and in a small control zone that has a smalllambda length compared to the lambda length of the secondary. It is alsomore pronounced in more powerful synchronous motors with a highinductive coupling between the secondaries and primary. Rapid changes incurrent when a secondary enters a control zone effectively imparts amomentary force pulse to the carriage thereby causing oscillations. Inextreme cases the oscillation may cause the secondaries to losesynchronism with the EM wave of the control zone and therefore stop thecarriages. This problem can be overcome by providing a rapid responsecurrent control system to react to the back-emf and inductance changerapidly, and compensate by increasing the available power to the coils.

Rapid response current control can be accomplished by the following:

providing a high available voltage to the driver switching transistors;

sensing a current indicative of the current output by the transistorsand determining the difference between the sensed current and a desiredcurrent level, and;

interrupting, based on the difference, the instructions for turning thetransistors "on" to thereby regulate the current output of the driver tomaintain the coil currents at the desired level. Additionally, thefollowing step can be included:

limiting the interrupting to a rate which is less than the switchinglimit of the transistors to thereby avoid transistor overheating.

FIG. 20 shows an improved zone driver similar to the driver shown inFIG. 8 of U.S. Pat. No. 4,675,582. The voltage control circuitry thatprovided a reduced voltage on second bus 531 (FIG. 8, '582) has beenomitted, so now there is only a single high voltage bus 529. Currentsensors 550, 552 and 554 have been added to sense currents directly inphase coils A, B and C respectively. Output from the sensors is directedto a new zone driver current control and gating logic 556 that replaceszone driver gating logic 520 in FIG. 8 ('582). Output protection isstill provided from 508 on line 510. The voltage setpoint and frequencyto voltage converter of FIG. 8 ('582) is shown in FIG. 20 operativelyseparated from 508 and is now used to set the current control setpointon line 558 to new circuit 556.

FIG. 23 shows the zone driver current control and gating logic 556. Onthe right side of the figure is shown the inputs from the A, B and Cphase current sensors. At the bottom of the figure are the 3 bits ofcontrol information for the A, B and C phases input from the zonecontroller along logic line 500. At the left of the figure is the inputfor the current setpoint for all three phases on line 558 and the outputprotection control on line 510. The output protection is present toprevent damaging current conditions that might develop from failed drivecomponents and is not the subject of this invention, so its operation isomitted for clarity. At the top of the figure, the output lines from thecircuit 556 are shown for the upper and lower transistors for each phasedesignated A, A and B, B and C, C. The current control of the inventionis accomplished along these phase control lines as will be explained.

All three phase currents are sensed and controlled independently, so theexplanation for one phase will apply to the other two as well. The Aphase current control is shown on the left side of FIG. 23. The phasecontrol information for phase A is fed in on line 506 and provides oneinput to the base of AND gate 560 on line 562. It also is inverted byinverter 564 on line 566 and provides one input to the base of AND gate568. The information on these lines 562 and 566 provides the desired,predetermined switching pattern for the upper and lower A phase powertransistors 530 and 532 (FIG. 20) respectively. Likewise, theinformation on lines 570 and 572 provides the predetermined switchingpattern for the phase transistors and the information on lines 574 and576 provides the predetermined switching for the C phase transistors.Depending on the actual current level in the phase, however, thispredetermined switching pattern may be interrupted by the currentcontrol circuit that provides the other input to the bases of the ANDgates 560, 568, 578, 580, 582, and 584.

The current control circuit will now be explained, once again referringto the A phase, but it applies to all three. The desired current levelreference signal is applied on line 558 as one input to the base of theA phase comparator 586. Within the comparator is circuitry for deadbandlimits for the reference current.

The comparator determines the difference between the sensed current andthe desired reference current deadband. If the sensed current is lessthan the lower deadband limit of the reference, the output of thecomparator is low; if the sensed current is greater that the upperdeadband limit of the reference, the comparator output is high. If thesensed current is within the deadband limits, the last output from thecomparator continues. The comparator output is one of the inputs on line590 to the two logic gates 586 and 588 of the A phase current controlrate limiting circuit 598.

Since a high voltage is always present on bus 529 to provide the desiredcurrent, yet a high voltage is only occasionally required, the currentsignal is frequently above limits. Therefore, the current controlcircuit is frequently turning the power transistors off and on to try tokeep the current within the deadband limits. To protect the powertransistors from excessive switching rates that are normally determinedby the L/R time constant for the phase coils, a switching rate limit isincorporated into the current control circuit. Excessive switching ratescause overheating when added to the normal transistor heat load and musttherefore be avoided.

The current signal to logic gate 588 is compared to the last signalreceived which is present on line 592. This OR gate comparison feeds anon-retriggerable logic circuit, 594. When the inputs are different, asthey would be for example if the current is "on" to a transistor and thelevel goes above the upper deadband limit, the chip logic feeds a signalout to line 596 subject to a timing circuit represented by RC elementsshown. The logic circuit 594 prevents the timer from starting over againdue to a state change until it has completely timed out. This timingcircuit 598 limits the switching rate that the current control circuitcan implement to thereby protect the power transistors from excessiveswitching at high currents. The output from NOR gate 586 is inverted andbecomes the other input to the base of AND gates 560 and 568. A typicalIC chip 594 useful for this circuit is a Motorola non-retriggerable "oneshot" chip, part no.

MC145388 from Motorola Inc., Austin, Tex. As mentioned, the currentcontrol circuit switching rate naturally seeks a level determined by theL/R time constant which varies the magnitude of the current, but thishas been limited by the rate limiting circuit provided. The result ofthis is shown in FIG. 24 where a typical time slot of current applied toan inductive load is seen. A voltage bus powering the typical inductionload is sufficient to drive a current through the load that exceeds thedesired current. For different operating conditions, such as forstart-up and for high speed continuous operation, the current level maybe set low or high or somewhere in between. For a low and a highcurrent, the current builds up and decays in about four time constants.However, for the high current the curve at initial turn-on and turn-offis much steeper than for the low current since the high current mustrise to and fall from a higher level in the same time as the lowcurrent. Rise and fall of the high current within its deadband will bemuch faster that the low current for the same deadband. Therefore, thenumber of switchings naturally occurring per unit time will be greaterfor the high current. When the same deadband limits are applied to bothcurrent levels, the results are shown for the low current at 543 and forthe high current at 545. As can be seen, the natural on-off rate for thehigh current is much more frequent than for the low current. The lowcurrent rate may be acceptable, but the high current rate will causeexcessive heating in the power transistors. When the rate limit logic ofthe current control circuit is applied, the high current trace willresemble the dashed trace at 547.

In operation, when the instructions on line 506 call for the current inthe A phase to be "on", the current rises due to the excess voltage onbus 529 until it exceeds the upper deadband limit, such as at 549, atwhich time the comparator output goes high on line 590 to rate limitingcircuit 598. If the time since the last comparator state has expired,the rate limiting circuit outputs a signal to the AND gates 560 and 568that interrupts the "on" instruction for the A phase current and therate limiting circuit 598 starts timing this new state. The current inthe A phase coils then starts to decay and drops down to the lowerdeadband at which time the comparator output goes low. If the ratelimiting time has expired, the "on" instructions for current can beun-interrupted and current to the coils will be turned on. If the ratelimiting time has not expired, the "on" instructions are stillinterrupted and the current continues decaying until the time hasexpired. In this latter case, the current trace resembles the dashedlines that rise above and below the deadband limits.

Referring to the dashed line trace, the rate has been limited to anacceptable level now, but the current swing is somewhat greater that theoriginal deadband limits. In effect, the narrow deadband limits for thehigh current level where the rate limit logic was utilized have beenbroadened to control the excessive switching rate, thereby preventingoverheating of the power transistors. If low currents are selected forthe drive, the narrow deadband limits may be the controlling factor indetermining the switching rate thereby allowing rapid response even atlow currents. This methods of establishing rate limiting is preferred tofixed deadband limits that must be readjusted for different currentlevels to be effective in limiting high currents without the samedeadband limits slowing the current response at low limits. This methodof limiting allows the drive to provide maximum current responsewhatever current level is selected for a particular set of operatingconditions.

The preferred circuits shown have been for providing rapid responsecurrent control in a transistorized inverter type drive havingpredetermined, remote source, phase control instructions. However, othermeans may be used within the scope of the invention for providing anexcessive voltage bus and limiting the current by interrupting thepredetermined instructions at a controlled rate. A simplification of thecircuit explained which employs only a single current sensor on thepositive DC bus, and omits the "AND" gates on the A, B, and C switchinglines to the transistors (much like the circuit in the '620 patent)would also accomplish the same control. With an effective rapid responsecurrent control circuit, the current to the coils of a control zone canbe rapidly controlled to keep the current at the desired level thusavoiding sometimes undesirable oscillations in the motion of the activecarriages.

The linear motor zone drivers in the tenter frame control system can allbe the same for the convenience of maintenance and parts inventory, butthey can be different for the first and second primaries. For the secondprimaries where phase and frequency matching between control zones isnot required, the drive powering a control zone can be a conventionalinverter-type adjustable frequency drive, with some form of currentcontrol. The conventional gating logic would include the means forswitching the transistors in a conventional manner. For the firstprimaries, however, the control zone controller with specializedinstructions for switching the transistors will always be required.

Zone Controllers

The control zone controllers in the tenter frame control system for thefirst primaries are all structured according to the zone controllers ofFIG. 5 of the '582 patent, previously incorporated by reference. Thesecontrol zone controllers include a first steady state memory such as 404to operate a control zone at a first steady state operating condition; asecond steady state memory such as 412 to operate the control zone at asecond steady state operating condition; and a third transition memorysuch as 408 to operate the control zone to achieve a transition from thefirst steady state to the second steady state operating condition. Inthe tenter frame, this ability to change from one steady state operatingcondition to another permits the tenter frame to change the MD drawratio from one continuous operating draw ratio to another whilecontinuing to draw the web of material without stopping.

Variable Draw Ratios

This particular feature, i.e., to be able to readily change operatingstates, i.e., draw ratios, for the control zones without stopping thepropulsion of the tenter clip carriages through the control zones,provides unique capabilities to the tenter frame of the invention.First, it allows the tenter frame to be started up, and the film firstthreaded through, at a low first steady state simultaneous biaxial drawratio and then, while continuously drawing the film, change to a highersteady state simultaneous biaxial draw ratio. Second, it permits finetuning the simultaneous biaxial draw ratio during operation to optimizethe process and achieve film quality not possible before since prior artsimultaneous biaxial tenter frames can only make incremental MD drawratio changes by lengthy and costly shut downs and start-ups of theline.

TD drawing of the film can also be adjusted and fine tuned duringoperation by intermittently driving adjusting screws, such as 907, whichmove the loops toward and away from each other. The tracks flex andslide at the loop inflection points to accommodate the angle changes.Such lateral adjustment features are shown in U.S. Pat. No. 3,150,433,which is hereby incorporated herein by reference. The screws and pivotlymounted nuts for adjusting the tenter frame width have a right handthreaded screw segment, such as 911, and nuts, such as 913 and 915, forone of the loops and a left hand threaded screw segment, such as 917,and nuts, such as 919 and 909, for the other loop. In this way rotationof an axially fixed screw, such as 907, moves the two loops in oppositedirections toward and away from each other. A motor, such as 905,attached to the screws and controlled by the operator accomplishesrotation of the screws prior to and during tenter frame operation. Whendesirable, then, both MD and TD draw ratios can be varied whilecontinuing to simultaneously biaxially draw the web.

At low MD and TD draw ratios it is relatively easy to thread up the filmwhereas at high draw ratios it is common to experience film tearing andbreakage with numerous film polymers, such as polyethylene terephthalatefilms.

After the film is running at the low MD and TD draw ratio, it ispossible with the apparatus of the invention to increase the TD draw byadjusting the tenter frame width and increase the MD draw by switchingcontrol of the tenter clips to a third transition operating conditionwhere the MD draw ratio is continuously changing until it reaches asecond steady state MD draw ratio. Then the control system can switchcontrol of the tenter clips to the second steady state MD draw ratio ofoperate continuously. This permits running at a high simultaneousbiaxial MD draw ratio which was not possible in prior art tenter framesbecause they operate at only one fixed simultaneous biaxial MD drawratio which cannot be threaded up when it is a high draw ratio. Highsimultaneous biaxial MD draw ratios are those that exceed 3X, orpreferably 5X, or more preferably 7X, and most preferably 9X. Thisresults in a simultaneously biaxially drawn film that was not possibleto make continuously in a single stage draw before. By single stage ismeant within a single draw section of a tenter frame.

Sequential Drawing

From the above it will be seen that in accordance with the presentinvention longitudinal drawing of a film may be precisely regulated dueto the use of a plurality of linear motor powered carriage pairsindividually controlled independent of other pairs with respect tolongitudinal movement thereof. Thus, biaxial drawing of the film may beaccomplished simultaneously as described or it may be accomplishedsequentially as well. In sequential drawing lateral drawing precedeslongitudinal drawing or vice versa. If it is desired to laterally drawthe film prior to longitudinal drawing, the speed of the carriages inthe diverging portions of the guide tracks is maintained constant suchthat only lateral drawing is accomplished and thereafter in thestabilization section the speed of the carriages is progressivelyincreased to longitudinally draw the film. If this mode of operation isdesired the size of some of the groups of coils as shown in the controlzones of the stabilization section of FIG. 10 would have to beredesigned so when the carriages are to be spaced apart, there is nevermore than one pair of carriages at a time in a control zone. In order tolongitudinally draw the film prior to lateral drawing, the guide tracksin the drawing section are readjusted to continue parallel and the speedof the carriages is progressively increased in the parallel part of thedrawing section to provide longitudinal drawing, and thereafter thespeed of the carriages remains constant as the track diverges in thestabilizing section to achieve only lateral drawing. Furthermore, ifonly longitudinal drawing is required with no lateral drawing, the guidetracks from 921 to 929 may be maintained parallel thereby not providingany lateral drawing of the film. Also, if only lateral drawing isrequired with no longitudinal drawing, the tracks would diverge as inFIG. 10, but the carriages would all travel at a constant speed withoutbeing spaced apart throughout the entire forward sides of the tenterframe. As long as all active carriages are traveling abutted at the samespeed there can be more than one carriage at a time int he operationalsection control zones.

MD Simultaneous Stretching Variations

The advantages obtained by the present invention stem substantially fromthe elongated stationary primaries divided into control zones whichcontrol zones operate on moving pairs of synchronous secondaries oflinear motors to impart predetermined speeds to active carriagescarrying the secondaries and tenter clips along the primaries. Thus,since each pair of active carriages is independently propelled (that is,precisely movable independent of the other carriage pairs) anacceleration program may be determined for superposing on the lateraldrawing a predetermined, controlled, longitudinal drawing. Accordingly,the longitudinal displacement applied to the film during its movementthrough the drawing section may be precisely regulated at all times;and, similarly, the speed of the film in the transport and stabilizingsections may be precisely controlled. Wherever the active carriages areto be separated, the control zone lengths are selected so there is nevermore than one active carriage at a time in a group of coils in a primaryof a control zone. Then, by predetermining the frequency and phasesupplied to each primary control zone, any desired drawing of the filmcan be achieved, such as varying the MD draw rate and the ratio ofamount and rate of MD to TD draw within the drawing section. Forinstance, the MD strain rate during simultaneous biaxial stretching ofthe film can be controlled at various positions in the drawing sectionof the tenter frame. The strain rate is defined as the following:

The strain rate, or instantaneous strain rate, occurring between twoadjacent clips at two different times during stretching is defined bythe following: ##EQU1## where: L0=length of unstretched film between twoadjacent clips at time=t0=0, at the beginning of stretch.

L1=length of stretched film between two adjacent clips at time=t1.

L2=length of stretched film between two adjacent clips at time=t2.

The average strain rate (ASR) is a special case of the strain rate wherethe stretch is measured from an unstretched condition at t0 to a fullystretched condition at t2. In this case, L1=L0; t1=t0; and t2-t0=totalstretching time. This results in the following: ##EQU2##

The strain rate can be controlled to be a constant value throughout MDsimultaneous stretching, or it can progressively increase during MDsimultaneous stretching, or it can rapidly increase and thenprogressively decrease during simultaneous stretching. Compared tocommercially known tenter frames the tenter frame of the invention canproduce strain rates two to three times greater. This is because for anygiven MD drawing distance and strain rate control, the tenter frame ofthe invention can run at operational speeds two to three and sometimesten times greater than any prior art tenter frame for simultaneousbiaxial stretching. For instance, commercial versions of the tenterframe of previously mentioned U.S. Pat. No. 3,150,433 can only achievecontinuous operating film exit speeds of less than about 500 feet perminute. The tenter frame of the invention can achieve operating filmexit speeds of about 1200 feet per minute. When drawing a film at thatexit speed and a 5X MD simultaneous biaxial draw ratio in about a ninefoot distance at a constant MD strain rate, the result is an MD strainrate of about 32,000% per minute.

In order to more fully appreciate the merits of the invention, polymericfilms were biaxially drawn simultaneously in the MD and TD directions onthe linear synchronous motor tenter frame of the invention using varyingspeeds, various draw ratios, etc. Although, in prior art machines,biaxially drawing film simultaneously in both directions has beenachieved, not until the drawing of film on the described apparatus willone be able to readily vary the absolutely simultaneous biaxial drawratio to obtain the outstanding properties of the films of the presentinvention - films that are biaxially drawn absolutely simultaneously inboth directions without prestretching in the machine direction, orbiaxially drawn in a precisely predetermined controlled manner.

Thus, films can be drawn at least 3X in both directions at strain ratesof from 10,000%/minute to strain rates as high as 60,000%/minute.Preferred films can be drawn at least 5X; the most preferred films canbe drawn at least 7X and most preferred can be drawn at least 9X. Thefilms can be any of the following materials: polyesters, e.g.,polyethylene terephthalate and polybutylene terephthalate, polyamides,polyacrylates, polyolefins, e.g., low and high density polyethylene,polypropylene, etc., propylene-ethylene copolymers, polycarbonates,polyvinyl chloride, polystyrene, polyurethanes, polyvinyl alcohol,polyvinylfluoride, polyacrylonitrile, polyimides, copolymers of ethyleneand vinyl alcohol, polyphenylene sulfides, copolymers of vinylidenechloride and vinyl chloride and copolymers of ethylene with olefinicallyunsaturated monomers such as vinyl acetate, methyl methacrylate, ethylmethacrylate, ethyl acrylate, methyl acrylate, acrylonitrile,methacrylic acid or acrylic acid and ionomers thereof.

Biaxially oriented polyethylene terphthalate film prepared by theprocess of this invention is particularly preferred herein and maypossess many unexpectedly good characteristics such as high mechanicalstrength, very low heat shrinkage, and excellent dimensional stabilitythat will make it an outstanding candidate for use as a base film inmagnetic recording tapes and disks, capacitors, etc. This should beparticularly true for those films biaxially drawn as high as 5X or 7X.

In drawing a web of film with the tenter of the invention, it has beendiscovered that the tenter, with active and passive carriages propelledat draw ratios exceeding about 3X and exit speeds exceeding about 200feet per minute, leaves a characteristic mark on the improved film madeby the tenter. This mark is evident on the thickened film edge bead asit leaves the tenter and is thought to be unique to this film productonly first produced by the process of the invention. The active clipsleave a stress pattern in the bead that is angled from the centralportion of the web toward the edge in a direction that is toward thedirection that the web was longitudinally drawn. The passive clips leavea stress pattern in the bead that is angled in a direction opposite thedirection the film was drawn.

In FIG. 15, the general direction of the stress pattern mark left by theactive clip is represented by arrows 988, which are angled toward thedirection the film was drawn represented by arrow 955. The generaldirection of the stress pattern mark left by the passive clip isrepresented by arrows 990, which are angled in a direction oppositearrow 955.

Tenter Loop Operation

FIG. 19 shows three predicted plots of clip carriage velocity versusposition for a simplified loop that simulates the tenter loop of theinvention. To simplify analysis of the system, the number and length ofthe control zones is reduced and gaps between return side control zonesare omitted; the stabilization section of the tenter is eliminated. Thepurpose of the three plots is to show how three different film stretchprofiles can be accommodated by the system of active and passivecarriage propulsion and in particular how the different profiles affectthe return side active and passive carriage deceleration and thecarriage stack. In FIG. 19, the dashed plot designates the EM wave speedfor each plot of carriage speed. Numerals 1, 2, or 3 indicate whichcurve the EM wave speed is associated with. Curve 260 is associated withnumeral 3; curve 262 is associated with 2; curve 264 with 1. Plot 260shows the tenter operating with a longitudinal draw ratio of (33.3lambda per sec)/(11.1 lambda per sec) or 3.0x; plot 262 a ratio of44.4/11.1 or 4.0x; plot 264 a ratio of 88.8/22.2 or 4.0x. FIG. 25 is adiagram of one representative loop of the simplified tenter that iskeyed to the plots to show the representative relationships betweencarriage speed and carriage position in the simplified tenter loop.

Certain portions of the speed versus position plot are predetermined bythe desired operating conditions for stretching the film. For instance,referring to curve 260, the entrance speed of the clips from 266 and 268which is also the stack speed is determined by the film speed; and theacceleration, final draw ratio, and exit speed of the clips, from 268 to270 is also dependent on film stretching requirements. These parts ofthe plots may take many values and forms not determined by the linearmotor system so they will not be discussed here. Referring to FIG. 25,the zero position at the left of the loop at location "a" corresponds tothe zero position on the horizontal axis of the plot in FIG. 19. From"a" to "b" is the forward side; from "b" to "c" to "d" to "e" to "f" to"a" is the return side. The first primaries extend from location "a" to"b" and propel the active carriages. The passive carriages are propelledfrom "a" to "b" by abutment with the active carriages or engagement withthe film, carried by the active carriages. The film is gripped at "a"and released at "b". From "b" to "c", the friction wheel propels bothactive and passive carriages. The second primaries extend from "c" to"d", and propel the active and passive carriages. From "d" to "e", thereare neither first nor second primaries; both active and passivecarriages are abutted here and preceding "d" and are pushed by thepreceding second primaries. From "e" to "f", is a segment of firstprimary that propels the active carriages; the passive carriages arepushed by the active ones. From "f" to "a", there are neither first norsecond primaries; both types of carriages are pushed through thissegment.

On cure 260, the carriages enter the tenter in an abutted stack at 266.At 268 is the stack exit where the carriages begin accelerating andseparating until they reach a final speed (33.3 lambda/sec) and spacingat 270. Shown is a drawn ratio of 3x. At 270 the carriages are propelledat a constant speed by the friction wheel before entering a secondprimary, hysteresis zone at 271. At the relatively low draw ratio of 3x,a large number of carriages are on the forward side so the return sidespeed must remain high to rapidly get the remaining carriages back tothe stack entrance. Therefore, in control zone 276 the speed of the EMwave is such as to continue propelling the carriage at 33.3 lambda/sec.In control zone 278 this EM wave speed is retained. In control zone 280deceleration of the carriages is begun by decreasing the EM wave speedto about 26 lambda/sec. The speed of idler and powered carriages dropsto about 24 lambda/sec before the end of control zone 280. So,regardless of slight variations in the rate of deceleration, allcarriages reach about the same speed before leaving control zone 280 andthe carriages enter the next zone traveling at this predictable speed.Therefore, the conditions under which carriages enter any hysteresiscontrol zone are a known constant, and slight variations in speed arenot additive from one control zone to the next. In control zone 282, thecarriages again decelerate upon entering the control zone and reach anequilibrium speed where control zone 282 is applying thrust to overcomecarriage friction before the carriages reach the entrance end of thestack at 288. It is important that the entrance part of the stack alwaysoccurs before the carriages leave the second primary. The actual stackentrance end may occur at a gap between hysteresis control zones orwithin one. The last hysteresis control zone is preferably always filledwith carriages so sufficient stack force is developed to compress thecarriage bumpers and therefore get the carriages on the proper pitch tomatch the EM wave in the beginning of the first primary control zone 261at 266.

In some cases it may be desirable to place a short synchronous controlzone at 286 in FIGS. 19 and 25. This corresponds to first primary 975 inFIG. 10. This provides a force assistance to the first few carriages incontrol zone 261 which are those putting the first tension on the filmat the tenter entrance. Control zone 261 corresponds to control zone A(coils 930/930') in FIG. 10. The two synchronous secondaries on eachactive carriage engaged in control zone 286 produce considerably moreforce in a compact space than the single hysteresis secondaries on theactive and passive carriages in control zone 282. This force assistanceis preferred since any force displacement of the first active carriagesin the transport section 918 (FIG. 10) to maintain tension in theincoming film, may result in slight stretching here which isundesirable.

Curve 262 shows the effect of a higher draw ratio, 4x versus 3x, on thereturn side. In this case, more active and passive carriages are shiftedto the return side so their energy is removed over a shorter distance toaccommodate a longer stack length. Control zone 280 is operated at alower EM wave speed (i.e. lower frequency from its drive) than it wasfor curve 260, so all pre-stack deceleration takes place in this controlzone. Control zone 280 and 282 operate at the same EM wave speed. Sincethere are more secondaries on the return side at this higher draw ratio,the stack length increases slightly as evidenced by the entrance end ofthe stack moving back along the loop from 288 on curve 260 to 284 oncurve 262.

Curve 264 shows the effect of scaling the line speed up whilemaintaining the same draw ratio as curve 262. As a result of the higherspeed, and therefore higher energy, of the carriages entering the returnside, deceleration forces must be applied to the active and passivecarriages sooner to achieve a low impact-speed before reaching theentrance end of the stack. Note that the percent energy dissipated ineach control zone is increased to achieve this. This percent energy isthe deceleration force F, which is the rated force developed on thehysteresis secondary, times the length over which deceleration occurs,delta L, (F×delta L); compared to the same force, F, acting for thetotal length of the zone, (F×L). If the control zones are unequallengths, but operated at the same current levels, the total energy ableto be dissipated in the longer control zones will be greater. Referringto curve 264, since the carriages decelerate over a longer length, deltaL, in each control zone the percent energy dissipated is increased. As aresult of this different deceleration profile, the stack end at 290 isshorter for curve 264 versus the stack end at 284 for curve 262,although both curves are for the same number of carriages on the returnside. At the extremes, the entrance end of the stack must always occurbefore reaching the end of the last hysteresis control zone and beforeentering any synchronous control zone; and it must occur some distanceafter the carriages reach the last equilibrium speed. The lastequilibrium speed is referred to as the stack overspeed. It shouldalways be high enough that the carriages can rapidly catch up to thestack but low enough to keep the impact when hitting the stack below alevel that causes damage to the active and passive carriages.

We claim:
 1. An apparatus for drawing a web of plastic film bypropelling individual carriages having tenter clips attached thereto atpredetermined speeds in opposed loops defined by a pair of carriageguide tracks positioned opposite each other, using linear motors, suchapparatus includes:first elongated primaries positioned opposite eachother on forward sides of the loops; second elongated primariespositioned opposite each other on return sides of the loops; a pluralityof active carriages guided around the tracks, each having a synchronoussecondary attached thereto positioned adjacent the first primaries forpropelling the active carriages, synchronously, in pairs, atpredetermined speeds, in a film processing section of the tenter frameto draw the film gripped by the tenter clips; each of the activecarriages further having a hysteresis secondary attached theretopositioned adjacent the second primaries for propelling the activecarriages in and through stack forming sections and into stacks ofcarriages on the return sides of the loops.
 2. The apparatus of claim 1wherein at least one passive carriage is positioned between each of theactive carriages in each loop, each such passive carriage having ahysteresis secondary attached thereto positioned adjacent the secondprimaries for propelling the passive carriages in and through the stackforming sections and into the stacks of carriages on the return sides ofthe loops.